this is gonna be one cramped (and HOT) chassis. i imagine the heat will stay in the unit (vinyl-covered foam being the great insulator it is) and that should result in some... interesting effects on the semiconductors
what if you built an external tube-driven reverb unit and just added a buffered effects loop in between the pre and power amps? that would be my choice if i were dead set on using a tube-driven spring reverb unit (in this scenario I'd much prefer my pedal though... hard to get hall/gated/modulated/etc reverb from a spring tank but i just gotta twist a knob)