318 to 340??

Please use the 340 exhaust manifolds on the 340. Then the exhaust will change. Manifold discharge is larger on 340.

Make sure the oil pan is same.
Not sure where this 340 came from.
Need the abody oil pan and pick up.
Drs side mount is different but you can use the 318 mount but you need to be creative withwashers, linger bolts and use grade 8 hardware.

Throttle linkage will be different and possibly a new longer reach throttle cable from gas pedal to carb. I will assume you are switching from 2bb carb to 4 bbl.

All minor differences. Good luck and have fun.