Depends on how you build your engine, but if you're coming from a fairly built 340/360 to a mostly stock 5.7 or 6.1 your drivability would probably get better. Power is probably pretty comparable too as a stock 5.7 starts right around 350 hp, a stock 6.1 a little over 400 (425 I think?). Go for the later year engines and those numbers bump even a tad higher. The fun part is that is just where they start and with zero mods. A few bolts on and you can easily get a 5.7 over 400 hp without any real difference in their behavior.
For your bushing question, do you mean any bushing that might already be in the crank or do you mean putting in one for a manual? If you're talking about the bushing that might already be there, you have to cut it out so you can install the register style pilot bearing/bushing. A die grinder with a carbide bit is the easy way from what I've seen, though I didn't have one. I spent the majority of an evening with a Dremel and chisel and finally managed to cut mine out.