Is it possible to take a stock plenum have it ported and polished to match a 70mm TB rather than buy an after market plenum?

 

why? buy a PP one t/b and plenum for $200

 

Its possible, but not much point. It the way the plenum does a 90 degree bend that the aftermarket one correct and smooth out.

 

^+1. All of the aftermarket plenums are 3/4" to 7/8" taller than the OEM plenum, this allows a larger and more graceful bend in the "elbow" that cannot be accomplished on the OEM unit with any amount of grinding.

The OEM plenum has a 70.7 mm bore on the inlet, so there's no need to do anything to match it up with a 70 mm TB.

In the last, if you have limited funds, buy a plenum first. Look at Accufab's flow data here and you'll see that any aftermarket plenum (except C&L's), with the stock 65 mm TB, outflows the stock plenum with a 70 or 75 mm TB.