Early Chrysler Hemi's cost
A LOT to build. Even with non-race parts. 392's are even worse. Everything for a 392 costs more, and there are a decent number of things that are unique to the 392 because it has a taller deck than the 331/354's did. Without rockers and valve gear you're not going to build it for less than $5k. Quite honestly, I wouldn't mess with a 392 unless I had $10k to spend on just the build. Seriously, just go over to
Hot Heads Research & Racing Early Chrysler Hemi Engine Parts and start pricing out some of the parts you need. Without the valvetrain you've got some money to spend before you even start working on the rebuild costs. Sure, you can build a 392 for less than $10k. But given the parts your missing, even a basic rebuild is gonna be hard to get under $5k.
In addition, you will need a decent amount of custom work to put the 392 in your '71. I've seen it done, but you're going to be on your own for a bunch of parts. You'd be far better off and money ahead building a 440. No, that wouldn't be as cool looking, but resale would probably be better since the 392 isn't an engine that normally goes in a '71 Swinger.