i have a base 1973 lemans not sure if engine is 350 or 400 has 2 barrel carb  will 1968 2 barrel heads equal more horsepower mobile dyno at atlanta dragway said 224hp 375tq

A 350 block will be stamped 350 or 50 on the block, drivers side above the oil pan. A 400 will be stamped 400 or 00 in the same location.

As for your read question, depends on the head. The most desirable flowing heads have a 2.11" intake and 1.77" exhaust valves. Pontiac made several over the years. However a good machine shop can do wonders with more common 1.96/1.66 heads with a port/polish job to make the head flow. What is the two digit/character designation on your heads? Should be between #1 and 3 cylinders.

block is a 400 heads are number 4h/4x 7:6:1 compression the heads i want to swap on are are 1968 number 15 heads 10:5:1 compression 

Actual #15s are from 1970 big cars. Still you definitely get a smaller combustion chamber at 87ccs vs the 98 or 114 ccs of the 1973 motors.