Machine 360 crank to fit in a 340 main block

Buying the correct 4340 forged crank, direct from the manufacturer would be the safest and cost effective way to go, My .02.

It'll be costly to get those journals machine that much, if it's done correctly.
Considering they would need to be turned oversize, then hardened and then ground/polish to finish size. Then all the radii...

There are other options.