I'd go with option 2. It'll give you some spares to work with on other projects. Also, if the exposed shells look too large or small, the enclosed power-feed belts should work fine. Assault cannons are generally modeled as having a large ammo hopper with a power feed anyway, so it should look just fine.
What loyalist said, defs option 2. I think I've seen option 1 around the place and yea they are a little too small I think.