Once I get it figured out, I'll post the size for future reference for anyone trying to do the same.
I noticed the same size differences between the sight/barrel screws and the receiver plug screws on the 7600 I just picked up. The sight screws are smaller than the receiver screws. Since Jeremy dropped the ball on reporting back the sizes ...Sounds good. I'll make it a sticky in this forum.
Sight/Barrel Plug: #6-48, Remington F202853 (I used a generic Pachmayr #21 Plug Screw with blue Loctite)
Receiver Plug: #8-40, Remington F14242 (I used a generic Pachmayr #22 Plug Screw with blue Loctite)
I got them here:
The Pachmayr screws are uniform diameter throughout and don't have the head/stop that the factory screws have. They can be screwed in so that they are flush with the receiver whereas the factory screws typically sit a little proud of the receiver surface.