360 Heads Reasonable?

X heads will have similar chambers. Magnums may give you a boost, but don't expect "instant race winner" with the stock low compression pistons.

You might be better off doing some basic prep on the heads you have now and replacing the pistons. Way cheaper than Edelbrocks and far better with your combo. Stock heads will support that cam just fine. Bigger ports are great for power, but generally don't help low RPM throttle response.

Rethink the double pumper with an automatic. Especially with a low stall converter (OEM 340 converters were 2200 RPM stall or so). A 750 AVS or Vacuum secondary will do you better.