The upper plenum, stock or aftermarket, has a 75 to 78 mm opening at the TB--there is no such thing as a "70 mm plenum". Also, the stock plenum is the weak link in the intake tract, if you examine
Accufab's flow bench tests you will see that any aftermarket plenum, with the stock 65 mm TB, will out flow the stock plenum with any size TB.
After market plenums are 3/4" to 7/8" higher than the stock unit which allows a wider (internally) passage through the 90° bend. Ford should have replaced the upper plenum when the PI upgrade was introduced--they did not as it's only real effect on the PI 2V is to roll-off the power a bit faster after 5500 rpm. If you were warrantying Mustang GTs you might not consider that as being a bad thing either.
Also and as stated by others above, 70 mm is the optimal size for any n/a 4.6 L 2V build--this is also spoken to on my Accufab flow data page.
In the last, the OP make want to read the "
Beginner's guide to bolting-on power gains" sticky...