I'm still wondering if you need a 5" downpipe on a 5.7 liter gasser? You'll need some backpressure in order to regulate the wastegate, or you could simply spiral out of control. I only run a 4" turbo-back on my 5.9 Cummins and it does great - good flow, low EGTs, 25psi all day long. All my Mopar 2.2 and 2.5 4-cylinder turbos have 2.5" full exhaust (turbo-back), and it regulates the boost just fine while allowing quick spooling to 30+psi. Trying 3" exhaust was too much, lots of wastegate flutter and no boost control. I even run cats (no mufflers) just to build some backpressure and help regulate.
If you stepped it down to a 3" or even a 4", you'd save a ton of room and/or have space to add some heat wrap to kill some heat soak. Or, possibly, switch to two smaller turbos - one for each bank. Quicker spool, and smaller plumbing required. I even remember seeing a kit that mounted the turbos at the REAR of the vehicle, where the factory mufflers lived.
I only ask because these are thoughts I've had about my pending Gen III swap into my '72 Satellite. I'm debating keeping it EFI and NA, with just a bigger cam; twin turbo (last choice); or Procharger (most likely, if I do a power adder).
