If you were to upgrade this 400 with my Poor Mans Racing Head.
Fully ported 194x 1.60 valve 305HO cylinder head you;d see how a 400SBC really goes. The finished chamber size ends up going just right 62-64cc
They flow a whole lot more than stock smogger heads.
Just right for the dished pistons and that cam.
if your 400 heads are the right casting you can do a very similar thing.
By generously home porting them and shaving them.
You'd be leaving the Big block in the garage.
The only trouble you will ahve is wiping the grin off your face
first time you rug it.
the performer manifold needs the heat risers blocked or restricited
a wood, divided carb spacer is nice too. The summit carb may need some rejetting and the accelerator pump calibrated.
You probabily poped the power valve in it. plugs are probabily finished.
the summit carb should work well once dialed in.