268 or 276 should be a good choice on the street with what you have. Did you figure out where you would be on compression with that set up ? That would help in making the final choice for me.
Good question. lol
I will pull the stock pulley off the 318 if I get a chance this week and try it out. it is supposed to. I have not gotten that far yet with the rebuild
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Pulling the plugs and isolating the problem is good advice.
Perform a cylinder pressure test while you have them out.
A vacuum test may help also.
A weak spark to one or more cylinders due to a bad plug or wire ?
Valve seal or valve guide problems ?
Good luck and keep us posted.
I think its fine , you should have a real broad power band that comes on early and idles pretty nice. Tuning shouldn't be a problem. 2800-3000 would be my choice on the convertor.
Once you figure out your thickness needed Buy a set of the Cometic MLS (multi layer steel) head gaskets and spray both sides with cooper coating,let them tack up before installing. This will help prevent water seepage when running alum heads.