It does not have to be precise to run well. Header are typical added for looks and sound.

To obtain maximum HP, you need to know:
* which RPM range the engine will be revved to, and size the collector length appropriately. Whether it is equal-length or not is unimportant for a streeter, but only the collector length is significant.
* the HP output of the engine,and size the diameter of the runners appropriately.

If it is a stock A15, is it an 80HP A15 or a 67HP A15? Generally the ones after 1982 are low power and do not make as much HP when revved high.