As far as a nice cruise RPM both the PGlide and TH350 are 1:1 in high gear so nothing is gained there. The TH350 however has a lower ratio in First than the Glide does in Low so acceleration is better.
The simplest engine swap for decent performance and good cruising would be the GM Perfomance 330hp crate engine. Being a small block 350 inch engine it is a direct bolt in swap to replace the 327. For extended highway cruising a TH700R4 or the newer electronic version both have the overdrive feature for lower highway RPM. Either of these is longer than the Glide so the driveshaft has to be shortened and the tranny crossmember moved about 2 to 2.5 inches further back.
Unless the 10 bolt needs major work it will be fine for a cruiser with a mild small block
You might hang onto that 327 particularly if it is a 300 hp job, the 300 hp and up all had steel cranks and were basically the Corvette engines but stamped for the body model they were installed in, and are highly sought by those who love the 327.
Just some thoughts......
I just picked up a '65 Impala SS.. It currently has a 327 with [i believe] a 2-speed powerclunk transmission (console says PRNDL).. My friend is looking at getting the car from me and making it a project/weekend driver with some pin-you-in-the-seat power.
Question is... what would be the best engine/tranny combo for this car. I'm going to yank the 327 as it's worn out, i'm sure (haven't started it but prev. owner says 'it just needs a carb to run,' but was sitting at least 10 years with no air cleaner judging from the old stickers on the car). Might as well do away with the 2-speed as i'm sure he would like to cruise on the highway at a reasonable RPM. If I was keeping the car for myself, I would probably leave the 327 and the powerglide but since it's for someone else..
Maybe a 350 with a TH350 behind it? Are there any pitfalls when selecting an engine for this car? The car has a 10-bolt rear right now. I haven't really looked at it much beyond that..
Thanks in advance.