1973 340, want more power.

Ether is fine. The 904 is lighter and uses less power. Most of the 904's are built with less HD type of internals than a 727, but that doesn't mean there not tuff or can not perform or be made superior. Most high performance minded guys will opt. for the 904 because of the above plus points. There about a 10th quicker in the 1/4 mile. This turns out to be a significant amount of power.
The 727 & 904 can handle and be made to handle a REAL good bit of power.