what is the highest you have herd of or seen? Ive been searching and this is the highest ive found.
How to Get 700-plus HP on Ford's 351C Engine - Hot In Cleveland

 

There is a 782.5HP 351C on You Tube.

Shazam made 799 with his 434(?) 400 based pulling truck, never saw a dyno sheet, but it was a serious build.

One of our Aussie brethren made right around 650 with a 2V Cleveland, no welding or port plates - ported IRON 2V heads!

I think Bob Glidden was over 800HP with his destroked 351C in NHRA Pro Stock.

 

And Tim has built many a 400 in the 600hp range. Lets not forget engine masters has been won by a 400 based engine for a quite few years running. Does the 400 make the most hp ever? No, but it does make more that most people could handle/want and provides a ton of torque to boot.