Exhaust/Header spec

Can anyone advise on the correct or optimum design spec for a pinto system ie.

4-1 or 4-2-1

Header diameter

primary pipe length

tail pipe diameter



I'm having one built from scratch and I'm thinking of going 4-1, primary pipes 35mm/1 3/8ths and tail pipe 47.6mm/1 7/8ths?



What are most running these days?



Thanks

Jim