MAM and secondary systems

Hey guys, just a few questions on multivector assaul mode.

I've been trying to work out how to make a ship that uses MAM, and as such I decided to deconstruct the Prometheus class and see how it was done, but I then realised it was way over budget on space.

The only way I could see it working was for the three ships to be built individually, with the MAM added on afterwards, but then realised that this would make the ships way under budget.

So I'm unsure how to do it. Also, how do you work out the size of each of the parts?

Vince.