In the same boat you are with your sight. I have a Lyman Great Plains Hunter in 54 caliber with a 1 in 32 twist for bullets.I've mounted a Pedersoli Universal Mid Range sight on my Sharps.Prior to that I had tried it out on hy .54 cal Hawken as my eyesight is going farther south every year past 60.I picked it up from Shooter's choice and Marstar also has good prices but they didn't have stock when I was in the market.I put a Hadley Style adjustible eye cup which is pretty much a must in my book being able to compensate for light conditions.I am having issues with the larger eye cup when folding the sight back and trying to clean or wipe the bore from the breech.I am going to try a longer Delrin rod that will flex the little bit I need to guide the rod in.It has worked with a spare I have for the Hawken but It is a little awkward being short.The sight seems to be fine for what I am asking of it but I have not had a look at really any others up close.The price does jump pretty good after this sight.I have also wondered about the Goodwin Style Creedmoor which is offset to make for easy wiping between shots etc.I did have to make a tapered shim to mount the sight vertical on the Hawken as it had a bit of a "list". This shim had to be put on the Sharps as well and still needs a little tweeking for that gun.Possibly not a common issue in higher end sights but apparently not an uncommon thing to deal with either.Hope this helps as my experience is limited to say the least.
No tang on a Lyman GPR??? Mine sure has one! And I mounted an old tang sight from a Savage 99 that I bought at a gun show. Try Buffalo Arms.Wait a second. The Lyman doesn't have a tang other than the little one on the breech plug of the barrel. So if your Lyman has holes from a rear peep sight they have to be holes in the one piece wood stock. And that means that the holes could be just about anything for spacing. Why not measure the distance between them and post it? That'll help us to help you.
This issue of how to mount a tang style sight on rifles with no tang comes up fairly often. But wood is a poor base to mount a tang sight's base onto. I think that if I had a rifle that I wanted to mount such a sight onto I'd go with an option of setting a steel or at least brass shoe down into an inletted portion of the wood to create a tang.
As an alternate I'd look at running a couple of the large steel threaded inserts into the wood leaving them slightly proud to act as pillar bases and then get the base of the tang sight ground or machined smooth to sit firmly on the inserts. Either way you really don't want to just wood screw a rear peep sight base onto a wood stock. The wood bruises too easily and the sight will never sit the same from year to year. It needs SOMETHING bedded into or onto the wood that spreads out the load into a bigger portion of the wood.
No tang on a Lyman GPR??? Mine sure has one! And I mounted an old tang sight from a Savage 99 that I bought at a gun show. Try Buffalo Arms.