Lt1
I've had an LT1 in my 60 for about 15K miles now :D . Stock 4spd w/ stock axles and gears. Speedo is off, but I don't care. RPM's on the highway are perfect for me...65-70mph at about 2500-2600rpms. I'm sitting on 33's. My plan is to run it till the gears go out (they've got 300K miles...
I've got a fresh LT1 (rebuilt out of a cop car) in my 60. It is the funnest thing I've ever driven. Also gets good Hwy gas mileage. 4spd around town seems to pull the mileage down a bit. LT1 isn't the cheapest option, but it's one that I've been pretty happy with. Thought about a TBI...