348's and 409's weren't particularly good engines 50 years ago, and time hasn't improved them. 409's in particular were notorious for cracking cylinder heads due to oddball water passages and hot spots. The low-compression 409 dump truck engines were a little more reliable, but finding one of those engines these days would be like finding a chicken with lips. If you want a big block V8, the 396/427/454 engines would be a much better choice, and less expensive to do a good buildup also.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!