1971 340's True HP?

I've read a few dyno tests of stock early 340's and they are indeed around 275 - 280 hp
the early 340's had a small 535 or 585 cfm carb so if you add the larger thermoquad you can probably expect a bit more.

The Carter AVS Carburetor for the 1968 -thru- 1970 '340' was rated at 630 CFM's.