The LS1 is the way to go.... no matter what you do you will always get more power with it compared to other engines with same mods.
the 02 engine is the best ...35% chance of having an ls6 block, ls6 style intake.
We did a conversion to a 61 impala.... the 01/02 exhaust manifolds are the only that really fit. There is a company in TX that makes awesome wiring harnesses (not painless...do not go painless).
$3,400 is a good price for an LS1. a built shortblock 346 is around $2,899. The going price is more like $4,500 for what you are looking for.
There is nothing terribly difficult about the swap at all.
Jon