Hi Chris,
The A15 GX head is a good move, this was the head that Nissan used for racing. There are several different chamber shapes and sizes but just the improved flow though the oval ports is better
than the stock round ones. The valve sizes should be larger and if your lucky it could be fitted with dual valve springs. To really make your motor run hard fit twin sidedraughts, 40mm is
big enough for street, Ph Dennis Marwood at Auto quip marketing in Auckland, he has the manifolds you need. Talk to him nicely, he the guy who built the Kiwi unique 1200 SSS. I would'nt
reccomend modifying the existing manifold as it ca cause cracks in the head into the water jacket and can be difficult to seal from vacuum leaks. 2 1/2" exhaust is really as big as you need,
Nismo extractors are that size and you can get every thing tucked up and out of the way on the standard brackets. If you do use stainless make sure it's the Magnetic 400 grade or better 304
grade don't mix and match grades or use 316 as you will crack with thermal shock from running though puddles etc. Don't forget a good set of extractors to. You will need different extractors
to clear the GX inlet manifold. Coby still have at least 2 different styles, also Tony at Marsh Motorsport in Auckland can source a
Nismo style one also.