70 318 with 340 internals/Cyl Head

3.91 + .030 = NOT A 340! It’s 323 cubic inches.
The 318 can use the crank & rods from a 340.
Depending on what piston is used and where it sits in the bore, zero deck, down .120, will vary the compression a lot.

“Some miles cam” is huge unknown and your asking us for a reading from a crystal ball. That is ridiculous AF!!!!

It’s output potential is limited by the heads flow ability. But probably hampered by everything else around it.

Sorry buddy, your question is a most joke.

A crystal ball reading with……. Unknown

Carb
Cam
Compression
Type of header
Exhaust
Items bolted on the engine
Ignition kind

At a 323 CID, IF this engine was actually built with some compression with a dome piston and the heads were very well ported, and headers that have a proper sizing to them, at a well built level of 1.4 hp per cube (which would be very good for an average guy in his garage) would equal around 450 hp.