how much more restrictive are the 1st gen downpipes over the secound gen downpipe?

 

sorry to bring this back from the dead but did you ever find what the problem was?

 

not sure yet still serchin want to do 1st gen downpipe with the stock manifold

 

I'd try this: Disconnect the exhaust system from the downpipe, in case something has gone wrong inside your muffler and partially blocked it. Test truck with load.

Perhaps your timing slipped?

I'm really leaning toward your wastegate being responsible here like you mentioned, but disconnecting the rest of the exhaust system would probably be a pretty easy check to do. It would suck to spend all kinds of time messing with the engine to then find out that there was a severe restriction further down the line in the exhaust system.

I'm a little confused about how you determined that dodge trucks can't handle the cummins...that has not been my experience. Admittedly, mine doesn't even have quite 300k on it, and it only makes a little over twice it's original rated hp and torque at the wheels, but it's done ok so far.