84mph is pretty darn good with twin 320HP.

The new hull is good for another 3-4.
How about shorties? Looking at the rigging shots, those drive are low in the water.
Maybe a set of the 375HP 6.0L GM small blocks and Imco storties?

how about the 383s from bam... +new hull +shorties and you'll be pulling 91-93mph. f'n respectable for a 100k boat.

Can they handle the Whipples ???

Hows about some Vette Zo6 motors. 500hp each!?!

http://www.mercruiserparts.com/383_Magnum.htm
Sure they can, same basic engine as the 320HP/6.2L.
My understanding is that they are reman 350s with a new stroker rotating assembly.
The idea is to replace the low output 454s.
Reman 454 blocks are more valuable turned into high performance like these 496 stroker engines.
http://www.1800runsnew.com/NLM/perf-engines-NLM.html

I don't know of anyone marinizing the 7.0L but it is surly is possible since Raylar does a stroker version and GM already has the 6.0L.
http://raylarengine.com/550_engine.html
There is some talk of the GM 8.1L big block engines going away. It would make sence that a 375HP/6.0L and a 425HP/7.0L would be the replacements.

Hp Is Not Equal To Torque

Well at 5250rpm they are equal...
For sure a 6.0L or 7.0L small block engine is not going to make big torque at low rpm like an big block 8.1L or 8.2L engine.