Both of these are real railway gun artillery, and were very devestating to what ever they fired upon. The Schwerer Gustov was an 800 mm cannon which was able to impact serious damage against it's target, usually being a large installation, such as a fortress or city. http://en.wikipedia.org/wiki/Schwerer_Gustav

The M65 was a German Krupp K5, captured by the US and made capable of firing atomic artillery projectiles a long distance (safe enough for the crew and gun to remain unharmed). http://en.wikipedia.org/wiki/M65_Atomic_Cannon
http://en.wikipedia.org/wiki/Krupp_K5

I would suspect that in game, these units would have a high maintenance cost, but on the plus side, have firing range of 3 hexes and indirect fire already on them. The Krupp K5 and Gustov would be unlocked at dynomite and the atomic cannon at the same time the atomic bomb is unlocked.

If you're thinking of how to move these without rail, there is one option, you culd have them like the Landkreuzer P 1500 tank, basically a huge cannon on treads. http://en.wikipedia.org/wiki/Landkreuzer_P._1500_Monster

On a side note, there are plenty of other weapons available that this mod could use, such as the Nebelwerfer, the USS Enterprise, K-19 early Soviet missile launching, nuclear powered submarine (I ask for this one of respect of it's crew men.), and many more.