If you went with any kind of fusion engine you'll have a lot more mass to work with. I was trying to keep this Karnov from going fusion. I did this without caring about C-Bill cost since they're way out of wack in BattleTech anyway. I care more about BV instead 
Basic SFE isn't much more expensive than Fuel Cell really.
Meanwhile you never have to refuel, so win for logistics there.
That said its less tonnage to work with so you might have to use the single C3M model idea I mentioned.
But, fluff bonus, you could use the Karnov-BA model that exists in canon w/ SFE as the "base chassis" and have it be a variant from there.
The 8 tons of BA space could be a C3Master & 3 tons for armor &/or Upgrading an ERML to LPPC?