1970 340 has the 1973 low compression pistons

In my opinion, as long as the whole package is tailored together, the low compression engine can be made to be a formidable performer. With today's rising fuel prices, and quality of fuel, a lower compression ratio with iron heads may be desirable. One thing I would do for sure is have a custom cam ground with a narrower lobe separation to fool the engine into thinking it has more compression by building more cylinder pressure.

If your stuff you have now is in good shape, use it. I certainly wouldn't be rebuying parts to change it. Make a plan based around a lower compression build, especially cam, and work with that.

Get in touch with a great engine builder and LISTEN to what he tells you. There are many theories and "experts" out there, be cautious who you follow. That said there are many very intelligent people on this forum, if you read a lot of stuff on here you will likely realize who to put stock into in short order. The builder of the Steve Clukey Duster is likely someone to listen to.