Spudman's picture shows one of the potential armament loads of the B-12. Another was with four SRAMs. to give you an idea of the benefits of speed and altitude, a SRAM launched form a B-12 would have five times the range it would have launched form a B-52. I'm including a cutaway showing that load.
Sorry, Elite, that model you remember is wrong. There's no place in the fuselage to put the missiles. Whoever designed the model was probably working from some photos, including one I've attached, where a designer who didn't do much research would have been fooled by the optical illusion of this angle looking like an off-center bay. As my other photo shows, the missiles were in the chines. I've also added another cutaway, showing one of the armament config.s of the production F-12B. This one is unusual in that it shows an M-61 in the forward port bay. I don't know if this was serious, implying that an F-12 would descend and try and force a target down by damaging it, or whether it was depicted to let potential customers know you could be macho and go mano a mano if you wanted.

The usual config discussed was four missiles (the YF-12A had three and the other bay was used for the fire control system).
Those missiles, BTW, were not the Phoenix. They were the AIM-47/GAR-9 enhanced and inherited from the F-108 program and came in both conventional and nuclear flavors. They gave birth to the AIM-54 Phoenix used by the F-14.