For me there were a couple of things. I hate to admit it but I own many different brands beside just this Plymouth. I built a 355 small block chevy myself with ported early LT1 heads a few years ago and got (just guessing - no dyno) a little over 400 hp for about $4000 in the motor. Chebby parts are cheap. After that build I figured besides the good feeling you get when the motor fires a crate motor probably would have saved me a few bucks (in parts and garage beers). This motor was going in something special to me. A 1968 Formula S 340 that is very nice. I wanted about as much power as I could get while opening the hood and showing what looks like a mildly modifiedy 340. That's why I went with the ported X heads and the 416 stroker. I may look into porting the original 340 exhaust manifolds and intake in the future and running a "stock appearing" class in the future.
I looked long and hard at the crate motors available. Really the real money I have in the motor I couldn't have done it much cheaper with a crate motor. A 450+ stroker will still cost $6000+