has anyone cut out the existing cat and replaced with 3" to the flange and then run a full 4" from the flange back. Im trying to figure this whole thing out and came up with this one but not sure about it

It would be easiest to just get the MBRP front-pipe and bolt it in place of the stock one which has the Cat. That way, you can put it back on for stealer visits.

You already have a dynomax so why not use Dynomax front pipe p/n 54194, about $130.