The first thing would be to see if the reciever is hardened to a higher degree. The old trick to do this without a fancy Rockwell hardness tester is to try a short cut with a file where the mark will not be seen. It just needs to be a short pass about an inch or less of movement in the file. The feel of it will tell you right away if you can drill the hole or not. If it has a grabby feel and cuts a little metal away then you're good to go. If the file skates over the metal doing nothing but removing some blueing then you will need to do some spot annealing or just give up. At that point even if you did get a carbide drill bit you'd need a carbide tap as well. And carbide tooling used by hand or in a hand operated drill press at these sizes is not for the faint of heart or short on experience and DEFINETLY no coffee before use. Solid carbide drills and taps of this size are as fragile as glass and shatter if you look at them wrong.
If the file digs in there's no reason not to go ahead and drill and tap. And from my own experience drilling into the threads without a barrel in place isn't a biggie. You just need to ease off as the bit breaks through into the threads and maintain the feed rate that the drill had during the full metal part of the hole. In other words you transition from feeding by controlling the pressure to feeding by controlling the speed as the transition point is reached just where you feel the bit entering the threading. You can practice this by going and finding an old threaded iron pipe coupling or other fitting with internal threads. Drill it full of holes with something like a cheapie 1/8 drill bit until you have the feel needed to do the job. Once you get the feel for it you won't need to worry about breaking the drill bit in your receiver. But practice makes perfect so I recomend buying a cheap 1 inch coupling from the hardware store to practice on for sure before tackling the receiver.
As for holding the receiver for the threading there's no jig required other than the drill press and receiver held in the drill press vise. Drill the holes first without moving the receiver in the vise. Then replace the drill bit with the tap. Dribble on some cutting oil and bring the tap down so it's entering the hole. WITHOUT POWER turn the chuck with one hand while using the other hand to give a little downwards starting pressure on the quill. If the vise starts to move then you may need to use your other hand on the vise and use your chin to apply pressure to the quill handle. This ensures that the tap starts dead on axial to the hole. Once you've got it started by a couple of turns loosen the chuck so you can shift the vise over and put a regular tapping handle onto the tap to continue. Repeat for the other holes. Remember, once leveled and aligned in the vise the receiver does not leave the vise until all the holes are drilled and tapped.
And if you have not tapped much before remember. 1/2 to 3/4 turn ahead then turn back to clear the cut by about the same then back to advance another 1/2 to 3/4 turn. And a proper cutting fluid is a must. Something like Tapzall works wonderfully but any proper cutting oil brand will be fine.
As for the hole location it's always best to keep the holes as far apart as possible and practical. Also most of my guns use double screws at the front and rear mounting points. You may want to copy that idea if practical with your setup. So two threaded holes about 5/16 to 3/8 apart at the #1 position and two more at the same spacing back at the #4 position. Of course the gap between the front and rear groups and the group spacing itself would need to be sized to suit your mounting rail.
The BIG issue will be to figure out some way to accurately mark out and center punch for the holes. THAT is where you'll want to measure and brainwork out a way to do this accurately. It's not streaching things to say that this initial marking out of the hole locations is more important than your drilling and tapping technique. So take your time and think about basic geometric principles, use any tricks or gizmos you think will aid you and double check everything twice (yes, that's 4 times
) before you actually mark anything.
The big one will be the initial center line that MUST be as axial as you can manage to the receiver center axis. One trick that allows doing just that requires your receiver to be a constant width for the whole length. If it is you can sit it on it's side and use something of the right height that is parallel for it's whole length sat alongside as a guide for the scriber marks for the front and rear groups. Or you can mount the receiver into the drill press vise with some aluminium protectors. But one of them is a length of aluminium plate that sticks up higher than the top line. Now you can use that as a guide. To scribe the center line you can use a cheap vernier caliper set to 1/2 your receiver width and use the internal fingers as a scriber to lightly mark the center line.
And obviously part of this setup would be to double check that the receiver is being held deap on vertical. Again this will depend on your setup and the features of the receiver as to how you ensure all this alignment.
Once you've done all this and you've got your intersecting center and cross lines break out the prick and center punches. The prick punch is an old toolmaker's tool that isn't seen much anymore. It's a sharper angle punch sort of half way between a scriber and center punch. It makes getting an accurate placement on the scribe line intersections much much easier. It is used with the lightest of taps so you just get enough of a divot to locate the broader angle center punch. Since finding one is darn near impossible you'll need to make one out of something like a nail set and grind a point that has about a 60 degree included angle.
As you can see there's a lot more to this to do it right than just drilling and tapping. Best of luck and enjoy your learning experiences.
