my 78 has a 373 from an 83 monte and my 73 (that is no longer in running condition) has a 323 rearend. are they interchangeable? and... when i run about 90mph im cranking out a good 5ks on the tach with the 373s which doesnt really matter, i built this 305 with some beefy poop and have pushed it well over 7k. so my question is, is the switch going to help this 3spd auto at all besides make it highway worthy again? i just ran a 14.6 last friday at the 1/4 (timeslip to prove for all you haters :-P ) im assuming this time will stay about the same, yay nay ?

No. They won't interchange. Changing to a 3.23 will deffiently help your engine live longer. Running at 5 grand alot is killing it. I am pleased with a 3.23 gear & th350 but someday I might change to a 700r or 200r.

what is the rpm's at 65-70? I am going from a 2.41 to 3.73. At 65 i am at about 2100-2200 rpm's. And why would ya drive 90 for any length of time. 14.6 1/4 you should only be at 90 for a couple of seconds...;)

can i steal the 323 pumpkin from my 1973 and just swap the guts of the rearend?

Afraid not, 73 is a 8.5 ring gear , the 83 has a 7.5 or 7.58.

yeah that answer ruined my night :( ddaaaaamnit. where can i get some 323s for this durned thing?

I found a set of 3.23s at a local swap meet a few years ago to go with my factory posi unit. I had 2.73s but it wasn't enough. Your best bet is find some from a s-10 or 82-85 camaro.