Hey Progrmr, I just had a peek at the Blackheart site and the the BH5H has 4,8 and 16 Ohm out's and the cab designed for it has a 16 Ohm speaker in her.
That being the case, you can basically go nuts and get whatever speakers you like and just make sure when wired up they equal 4, 8, or 16 Ohms. Then plug into the corresponding output on the amp.
A little theory lesson too. You said "I assume low impedance and as many speakers as will fit in the car??"
Matching impedance is FAR more important than the actual impedance.
Looking for the lowest impedance speakers possible is an activity reserved for solid state amplifiers that can conceivably deliver more power into a lower impedance speaker. Think car audio amps and sub woofers. That kind of thinking shouldn't be applied to tube amps. Mismatching impedance in tube amps will always cause a drop in perceived power and a change of frequency response. (Note: A couple of weeks ago I was jamming with mates and was complaining about my lack of grunt and muffled tone all night. It wasn't until I got home that I discovered I forgot to switch my amp from 8 to 16 Ohms to match the 4x12 I was using that night. To Peavey's credit, I ran my 60W tube head dimed for 3 hours like this and it didn't bother it at all, but it did prove to me that my theory was correct by sounding like I had a massive blanket over the whole thing)
The part about as many speaker as as you can fit is spot on though. More speakers=more surface area=more SPL generally speaking. One of the guys I stalk on Youtube has a video of the Marshall factory where he plugs a Class 5 combo into a full stack and it sounds EPIC.
