I need some schooling on Gen 3 Hemis differences. Power v Reliabilty v $$$ v Fitting into a '68 barracuda fastback

Depending on what transmission you go with will determine how much tunnel work will be needed. The slant six k member will work. Better fitment would be Reilly Motorsport Alterkation coil over setup. Or Hemmy Denny setup. You will have more room than a 440. You won’t need fender well headers for sure. I have a low cost radiator in mine cost me $148 and has been in mine for 14 years now. With a tauarus fan. Mine is a stock 04 5.7. Not that much hp. But great for what I do with it. Have over 125,000 miles on it in 14 years with no problems. I used the 545rfe trans so had to cut the tunnel. I’m guessing you will be going with a manual transmission so there will be some cutting involved. I think the 727 is the only one you can fit with the swing of a hammer on your tunnel.