Extra hp out of my 340.

If you're already at 340hp, without headers and without an alloy intake, then it would seem someone has spent a lot of time on your heads. It would seem a shame to take them off.
So in that case, it makes getting 400 a little tougher, and probably the end result will be a much worse combo. With headers and a matched intake, it would still take a cam in the neighborhood of 2 sizes bigger than the current one to hit the 400. And it already takes a big cam to make 340 without headers and intake.
So the bottom line is that the new combo, with bigger cam and resultant lost cylinder pressure,will give up a lot of bottom-end torque, and turn your current strong bottom end into a bit of a doughy goo. The 3.23s would have to go, unless you don't mind slipping the clutch out to an extra long ways. But in that case, you're gonna need to think about the clutch.
From experience, I can tell you that 400hp wants a starter of between 10 and 11:1, and that is about 3.91 to 4.10s with your 2.66 low gear.
So, how's that expression go?..... It would be kindof like cutting off your nose to spite your face.

I solved my problem with a GVOD, and a SMALLER cam. I ran 3.91s and .78od;problem solved.
For you; 3.91/3.23= plus21% more starter gear, and
3.91x.78od would be equivalent to a 3.05 final drive, which being 5.6% less than your 3.23s; 65mph would be 2460rpm with 27" tires,versus, 2610 with 3.23s

Yeah, I know, big coin. But I've got maybe 115,000 on that unit now, so in the end it was the right decision for me. That car used to be my DD,Daily Driver.