In my opinion the easiest bolt in swap would be a later model 235/261 55 and later full pressure engine. Change the water pump to a short shaft (Or use the Adapter plate available from some vendors), relocate the bell crank bolt (carefully) to the other hole in the block and drill the front mount so it's like the earlier engine and drop it in. Those engines are just as reliable as any V motor (more so than some) and should be more than enough motor to tool around town and mellow speeds on the highway. These motors are typically plentiful on CL and usually you can get one someone is pulling from a running car or truck due to V-Motor bias. The later 250 style sixes and V8 swaps are very doable but are going to require much more work than the late early sixes that are pretty much dimensionally the same. If the exhaust is still there it will likely bolt right up. GM designed those blocks to be used as replacements for the earlier motors (Hence the extra bell crank hole).
If when it's together he still want's to go faster then there's tranny swaps and rear swaps that will get him there. But if he want's to go "real" fast then there's other stuff like brakes that should be worked over first.
When I slap on my Carnac hat and lean towards the screen to get his interpretation of "original but reliable" I think what he's looking for is not necessarily "Original" but "Period Correct" or "old school" as in he doesn't want to big an anachronism in his engine compartment. But once again that's my interpretation but I am a little Psychic at least when I'm wearing the Carnac hat.
Or is that Psychotic, maybe I should take my pills...
MONGO
