What engines/transmissions are available in the donor cars?  What car is
this going into?  A few points to consider:

If that '73 Impala has a 400 small block, it will get crummy fuel mileage
and they aren't exactly powerhouses (although they make great cores
for hotrod buildups--toss in a 350 chev crank for a high-revvin 377).  
They also tend to overheat if they've been
rebuilt, and (especially 4-bolt blocks) are somewhat prone to cracking.

If you've got a 307 olds kicking around, they can run damn near forever,
but again, it won't be a hot rod.

A transmission from a Chevy will not bolt to a Buick/Olds/Poncho motor
(and vice-versa)
unless you've got one of the (relatively rare) dual-pattern bellhousing
'boxes.  Also, if you switch from a TH350 to a TH400 (or vice versa) the
crossmember will need to be relocated and the driveshaft won't fit.