L29 heads (I believe are 279 castings) are good for 450 max HP NA in my opinion with some light porting. The ski jump and the small valves really restrict flow over 4800.
But a cam, intake, exhaust, and some light porting on the heads got me a 10 mph gain on a 7.4 MPI in a 24x7. Also, I spin it to 5400 with no problem on the stock bottom end. The gen 6 block is a great starting point for any real-deal build later on as well.