I've had a couple built by a local muffler shop. A bit pricey, but if they were available in the mass market they'd not be cheap, either. Seems I paid around $250. For the last one I took into the shop the Y and straight section off a 454 fullsize and had them bend and weld on the 2 down pipes. When the truck rusted to death last year I kept the engine and that pipe. It was made from heavy walled pipe. Going to mod it to fit my current project. Right now I have a 377 breathing thru those little 1-7/8" stock pipes and I know it's hurting HP.
Then I'll be ready to drop $7-900 on a dyno tune. Ouch.
