Winchester 490 came available with 5 or 10 round mags. Seems obvious here what we have. It could be the magazine catch is 180 degrees out of whack. If it's properly positioned the magazine catch should hang down with the bigger or longer pointing down and slightly forward. If it's spring is properly positioned you should be able to push it forward and when you release it, it should pop back to it's first rearward position.
If it does not do this the spring could also be accidently positioned backwards.
If this small test checks out okay, I suggest you try another magazine. preferably NIB.
Also check the magwell is screwed tightly into the receiver. If there is loosness here it may allow play which could contribute to the problem.
I don't know if you've done a complete tear down, so I'll say this right off, when taking the barrel from the receiver, there is a little "U" shaped yoke that secures them. Be very careful not to damage the ejector which is precariously pinned to the back of barrel. It's very easy to damage it, especially on reassembly. Treat it gently or it's damage will cost you in hard to find parts buddy.
It's also possible the trigger mechanism is loose as well, as it's a bit of a chinese puzzle to get the rifle guts back together correctly.
Winchesters are grumpy and require at least three different screwdrivers tips for a complete teardown IIRC.
Two screws secure magazine floorplate, now lift barelled action from stock, two screws secure black housing, two more fasteners secure the magazine release and back of action now you can lift the aluminum block away and you should have left in your hands the trigger mech screwed to the receiver. Now you can plainly see the barrel yoke just under the front of the receiver. If you go further watch that fragile ejector I mentioned previously. With the barrel off, you can push the cocking lever forward and the bolt will follow for cleaning disassembly. Watch not to lose the firing pin as it just sits in place at the top of the bolt.
If these small tests work out, chances are it's the magazine. Some Savage mags are very very close. When you got it was it from an original owner? Or does it come from a long line previous?
Also I've had very old magazines come apart on me. The lips from use and abuse over the years just spread and the mag refuses to feed correctly. Eventually the ten rounders just popped the weak spot side welds and the magazine was a total loss. It took 11 years of gopher & small game hunting/shooting in Saskatchewan to do this.

For some time before that causing jamming issues just like yours. Buy some new mags buddy! Bet that's your problem.
Edit: I almost forgot. Does your serial number have an askerisk star behind it?
There was a recall a very long time ago to address this rifle could possibly fire out of battery with a round in the chamber when not intended.
The star denotes it was reworked by the factory.
Now you know.