I take it this is a streetcar. I have a 418w in a similar application. I had a Performer RPM professionally ported with the runners balanced. It made a big performance difference on the dyno. Either manifold will acheive your goals. The 1 5/8 headers will hold your motor back. FPA 1 3/4 full length headers are awesome on my car. I replaced my exhaust with the 2.5" Magnaflow mandral bent true x system. Great sound and better seat of the pants performance than my previous flowmaster 3 chambers with 2.5" non mandel bent H pipe. I would not go bigger on the pipes than 2.5".