I'd just stick with the 400 cubes and spend the money on heads, cam, carb and intake, ignition, etc.
Where your going to get the most gain per dollar. It had 9.0:1 compression, "194" heads, a lazy cam and was only available with a 2bbl and single exhausts. The only reason the trucks were rated at 205 hp-20 less than the Mustangs is the trucks used single exhaust and the Mustangs had duals.
The 302s are the same roller-cammed model used in "5.0" Mustangs. The same goes for making a 383 / 400 Mopar into a 451. It requires all kinds of custom machine work,-including relieving the block for clearance- and the parts are expensive. The buff magazines all tell you to put a stroker rotating assembly in it and make a 455.
( Or a 461 / 467 if the block is bored .030 or .060 over ). For $1800 you could buy a complete cam kit-cam,lifters, springs, timing chain, rockers, pushrods etc-from Comp Cams or Crane or Lunati, an Edelbrock carb and intake, a set of headers, and still have a little left over for rings, bearings and oil pump and maybe minor machine work-i.e.-turning the crank .010 under, re-sizing the rods etc.
If the short block was in good shape-you could build your whole engine for that.
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