I would like to build my own top mount mani and downpipe. My boss is doing the welding. Where do I get the material to build one? And what size does it need to be? It's a built b18c1 looking for around 450-500hp.

 

Ace Stainless is a popular place to get the weld els and stuff from, you'll also need flanges. I've bought stuff from both Weirtech and Brad Campbell (both members on here) and been very happy with them. Weir also sells stainless weld els at a good price, and he can cut flanges. Brad will also do flanges and can laser cut you a collector if you don't want to make it yourself, his prices are reasonable and IMHO worth it considering the time to make a PERFECT fitting collector especially if you don't already have a jig or something for it.