If the truck manifold is really 2.5" , running a 2.5 "J" pipe,you can make quite a bit of power.
The problem/design flaw w/the stock manifold is not having equal flow from all the exhaust ports, sharp 90 degree bends,etc..
There are plenty of people that have used a ugly log type turbo manifold & have made a lot of power. IIRC, Leo's 292 turbo ex manifold is not too pretty & is basically a log type manifold & his engine makes good power.PG. 143.Power manual.
I believe he could make more power from a better designed manifold,but like I stated earlier, you can make good to great power even w/log type turbo ex manifolds.
The record power from a 250 CI turbo L6 w/a SPA turbo manifold is around 780 HP,single turbo. The SPA manifold is pretty well designed,but for all out power,a tubular ex turbo manifold will make more power.
As 56er stated,guys can make a lot of HP & more torque w/stock Syclone/Typhoon manifolds,around 740-750 range on pump 93 octane fuel. BTW,the Sy/Ty manifolds look & are very restrictive,but up to a certain level oh HP they still work fine. Let me post a pic if I can find one,,,,,hmmmmm, http://www.syty.net/forums/showthread.php?t=42926&highlight=exhaust+manifold+pics&page=2 3rd pic from top of pg. Just 1 side to give you an idea ,plus the ports do not match up to the heads,turbo flange has a mismatch also.
The turned around ( FWD facing ) LS manifolds look almost like headers & many people run those & weld on v-band flanges to bolt on turbo/s
