the cam phaser can be locked out for free, well pennys if you dont have anything laying around the shop, just slide the back cover off and stuff something in the cavaties to keep the vanes from moving and put the cover back. It is a little more tricky to limit it but to straighht up block it is very easy.

I would just buy a 6.4 used, drop in better rods and piston keep the stock forged crank to save $1000 or you could sell me the crank for a couple hundred and put that towards an aftermarket stroker crank. You get the best heads this way that you can massage yourself, your gonna spend at least $1000 on a new cam and valve springs and a butt load on the tall drag pack intake, this is gonna be the big dollar stuff to make the swap, you could save a ton by having the stock cam reground ($100 VS $700) if there is enough meat to get the grind you want. Chances are the block could be used as is, not many high mile 6.4s and when they do get tons of miles they still have almost no bore wear, typically clean up at .010 over. At 392 CID it will be very happy to rev high enough to make 750 HP, of course bigger is better but not needed here. The 6.4 has the biggest bore and works best with the apache heads. I think this is your best chance of gettin 750HP under $8000.

I got a machine shop tooling up some parts for me to address oiling issues at high RPM on the new hemi. They would even be a nice upgrade for stock swap hemis, I'll make a post when they are ready to sell, the first ones are being tested right now with great results.