I've read that the performance advantage from dual manifolds/split manifolds is maximized after about 3 feet of pipe- after that two pipes/two mufflers are for looks and sound. If you want two pipes you should 'x' them at the cross member. If not, 'y' them at the cross member.

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi