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+2 for the 406 SBC. I've never understood the 383 attraction at all. 450 hp will me more easily obtained with the x-tra cubes. You could even use the factory nodular iron crank at that level and still be ok.
The attraction is kind of a holdover from when the 400 had an undeserved bad reputation. I think folks also get overly focused on keeping their existing 350 block.
Definitely true that it's HP/CID that costs money..the lower the ratio, the better the $/HP.
It is getting harder to find decent 400 blocks, although the aftermarket has filled in nicely with some very high quality products at a price that makes looking for a used block for any serious power goals kind of a waste of time.