My car is ‘71 GTX (RS23U1G…). Fender tags (2) are present, but I don’t have the build sheet or the window sticker. Is there a way to determine what wheels and tires were original on this car?

Fender tag won't help.
Just the broadcast sheet and window sticker.

^^^ Correct.
IF the car is a completely original car, a little detective work can help:
Check the spare tire & wheel: black wheel with wheel-cover scratches could indicate it had full wheel covers.
Same for the dog-dish or "poverty" caps.
I'm not certain if that year had body-color wheels for poverty caps and black wheels for full covers.
If there are traces of red paint on brake drums, that would indicate "road wheels" (aka Magnums).
I'm not sure if that also applied to the Rallye wheels.
Tire size may be written on underside of trunk lid, sometimes with R (red-lines) or W (white).
I'm not sure what the callout for white-lettered tires would be.
Google images of untouched original cars, magazine test cars, or the ordering info at Hamtramck-Historical might help too.

Thanks for the replies! You’ve confirmed what I was thinking, but I thought there might be something I was missing. My car was a rolling project with no engine, transmission, etc., original wheels long gone. Today it a running, driving car. I put a set of reproduction 15” rally wheels on it. I guess that will be close enough. Thanks again!