First off, that's a sick price dude, the .50 Caliber + is one of my favorite Boogies so far, I've had a IV, III Blue Stripe and a C+ and aside from the C+ I think the .50 Cal+ is my next favorite - mind you this is my opinion based on strictly the Lead channel and used for METAL!
Anyhow, my .50 Cal+ is a combo and it measures 17" wide which is the same as a Mark series Medium/Long Head, so you should be able to fit it.
FYI: I've been thru a few a different quite a few tubes in the .50 Cal and so far I am VERY happy with:
V1-Tung Sol 12AX7
V2-Electro Harmonix 12AT7
V3-Tung Sol 12AX7
V4-Tung Sol 12AX7
V5-Tung Sol 12AX7
Groove Tube 6L6's for Power Tubes.
I have some pretty cool metal settings if your interested, not sure your style but just thought I'd mention it.