Ah yes, the old extra 421 hole. Good one!

When aluminum w/p's 1st came out with stamped steel impellers, i R&I'd the factory iron impellar.
Yet to have to do the mod, but havent bought any w/p's since the early 90's.
Have 4 good old stk ones left.
Now that i said that...

And plugging the pypass hole between w/p hsg and intake x-over.
Drilling two or three 1/8 holes in t-stat.
Detach frt x-over and underside heat x-over from all intakes.
Fill exhuast to intake maniflod in heads.
Shim oil pump spring to get extra 10lb's pressure.
Tack weld oil control and drain back baffles in stock pans.
Grinding smooth all casting flash in blocks and heads.
Did i mention drilling a .030 hole in rear plug to oil dist gear?

Enough for now, need to get those parts prices.