Looking to know if a 1990 G20 front end would fit on a 91 GMC G10 rally?

That's a pretty good question! It looks like the core suport my be different if the headlight bucket area. Exterior dimensions should be the same. Go for it!!

I don't see why not. Just a different grille design, but should bolt up no problem.

Yes, everything will fit just fine. Just make sure you get all of the correct brackets and hardware. You may need to do some wiring though to make the headlights work correctly.