Which heads wpuld be better?

R/T's flow stock what the best x,j head does ported.

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Only on the flow charts at sheadydell. Head flow varys alot from head to head. Ryan will tell you that. He'll also tell you the a X or J etc.. can be ported beyond his listed numbers. 270 cfm is possible without getting wacko. Of course this will depend on the actual head itself.
Also some R/T's tested at other places like Hughes for example flowed alot less than advertised.
If you can get a head cheap enuff, then it could be worth the dollors and sense over brand new heads. Of course this falls under all things pending. Where you want to be vs. money in pocket. What one is willing to spend vs. performance gained.
Dick Landy's shop had a demo on highly modified head that went over 300 cfm. However, in order to reach that level, I would call it above and beyond the call of duty and sanity to achieve.
Welding the port and moveing the port roof to the valve cover line with making a new roof with I think it was A-B puty or something like it...

Just do not ever race flow bench vs. flow bench.
The real deal is what you can get from a head on the same bench, same operater, same technique.