Would it really affect anything to leave the top 2 bellhousing bolts out on a trans install as long as the other 6 are in tight?

 

I left the top one out and never had issues.

 

Well its certainly best if you get them all in, However I will say GM ran the transmissions with only 6 bolts for many years

 

we used to just run 4 in a racecar, the last 2 lower ones on each side(vert.)
never saw a problem with it, but i wouldnt rec it-most gen 3 engines only get 5 if using a 350-400, or older 4l80e
the lower ones are the most important, as theirs no support bet. the engine/trans mounts, it would want to sep. if it could