what needs to be changed in a 72 340 to be like a 70 340?

The Dougs have the fit and quality of a TTI without the price:

http://www.summitracing.com/parts/DOU-D453-R/

This article has some good info on how to build a mild 340:

http://www.compcams.com/Community/Articles/Details.asp?ID=-738676943

If you have the money I would go with a 4" stroke crank. Don't buy a balanced kit, have a good local shop balance it. If budget is tight just go with the good KB pistons and have it balanced.

And the X heads aren't any better then the J heads other then the bigger valve which can be put in the Js (some Js had the big valve but that is just a machining difference the cast is the same).