I understand your line of questioning and it makes perfect sense to me.
As long as the current exhaust (SSAC etc...) is functional, it might hold you back but won't hurt anything (per Dre) to use it to get started.
New fancy-ass custom header = $1,000.00+ or so....(?)
New 3" exhaust, res, muffler = $500.00 or so...
That adds $1,500.00 to the "price" of a big block VE. With plenty of folks on a budget and trying to "work this" into their pocketbook.
*Existing Decent Exhaust* = $00.00 Already on the vehicle, working for now. Will still allow the (big block) engine enough exhaust flow to produce and estimated 235-240 whp per Dre.
You could always "uncork it" and upgrade the exhaust with next years budget.