How to get 400 hp out of 1971 340 .30 over

Is there a reason that you want or need 400hp?
I mean 325>350 is already a ton-O-fun on the street,
and
with only 3.73 gears, you'll need some bottom end torque, which will go away with a bigger cam....... requiring a higher stall ....................
What you didn't mention is your convertor's stall speed, which with a 340 and 3.73s, will likely need to be over 3000.

As for a bigger cam, each 6 or 7 degree bigger cam, moves the power up about 200 rpm, and steals cylinder pressure off the bottom to get it. Your 268 cam is probably already peaking at or near 5000 rpm.......... which with 3.73s
is 40 mph in First, and 67 mph in Second.
Is that what you want?
Or are you like me, I want the power from;
take-off/zero mph, to ~40/50 mph, not very often after that. If that, then
you will need one of several things;
1) a higher stall
2) more Torque-Multiplication, either in the back or in the transmission
3) more cylinder pressure
4) a lighter car
5) a longer stroke
6) or some combination of those.
7) or ..... forced induction.

The purpose of ALL OF THESE is to increase the ratio of; torque at take-off to pounds of car. Leave the spare tire in the garage ............................