what crankshaft could i use?

See this car right here, it has a steel crank out of a '65 318 poly. It worked fine for the 727 converter.
I have lots of these cranks because people throw out these motors. I ask them if I can have it and they give it to me. I pull the cranks and toss the rest.
Your motor will have to be re-balanced because the Poly cranks are lighter (Oh Damn a lighter crank) I had to add a little weight to two counterweights even with the lighter Eagle SIR rods & old NOS TRW pistons (which are heavy in todays standards). So if you use the stock rods you will propably have to add more weight or get a lighter piston.
If you are only going to sling it to 5500 and 400hp I wouldn't worry about the cast crank.

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