There is a reason (well several actually) why so many people put sbc engines in their rods. Because they are so plentiful and popular you can build one cheaper than just about any other engine. On the other side of the ledger you have all the other motors, with engines like Gastricks Hemi right at the top costwise. He can probably tell you from experience that every part he bought cost 2-5 times as much as a comparable Chevy item. Same goes for Buick, Pontiac, Olds, etc. I have about $ 3000 in a rebuild and mild hop up in the 394 Olds engine in my RPU, and I still don't have it running. I bet Bill's wallet is a lot lighter after playing with that big boy in his Deuce too. I have played around with Ford small blocks and big blocks, and even they can cost a couple more times than what a similar Chevy costs to build.
Second factor is what goes behind the engine you pick. Most of the "oddball" engines (and I use that word as a term of endearment
) come with goofy transmissions that are not all that desirable in today's rodding world. Bill had to buy an adapter to put a th350 behind his Hemi, and I had to buy one to put a th350 behind my Olds..............to the tune of $ 1000 for the adapter setup, not counting the cost of a built th350.
None of above is meant to scare you off from doing a different engine, but simply as a reality check. Even the lowly flathead can cost a few bucks to build. Tony DiCosta built the one that was in Total Performances T bucket a while back, and sold the motor later on for $ 9000.
SBC engines are a little overdone, but because of the above reasons (plus the fact they are dependable and run great) they can be a wise choice. I hadn't played with a SBC in over 30 years, but after putting one in my 23 I see why people like em. It never fails to fire, runs as fast as I will ever need, and just keeps on going mile after mile.
Finally, I like it when an "oddball" car (again using my term of endearment
) like your potential Desoto uses a similar brand engine. For example, I hate seeing a SBC lost in a big Buick engine room..........a nailhead or later Buick would look so much better. So a Hemi or 383 or 440 Mopar would make your Desoto a little cooler, as would the flathead 6 you mentioned. I don't know if you would be happy with how a 6 would pull that sedan around, but that is a different story altogether, and a personal choice you have to make.
But it sure is a cool sedan, hope you get it.
Don