user1453, are you really sure the upper half of the H-bridge does what you think it does? In order to turn the mosfet on, you'll need to supply 50V + 12V to the emitter followers. At present, Q3,Q4,Q7,Q8 are only ever going to see at most 12V. If this were a real circuit, those mosfets would've probably blown their gates - followed by some more smoke.
Look up voltage translation circuits. Open-collector/resistor pull-up types are probably the kind you want, although AC coupling via capacitors/transformers would also do.
One side note - you're probably also better off using as many "ideal" spice components as possible - AC voltage sources for the (I think it's an oscillator, but I haven't built any in a long time), and get rid of the voltage divider op-amp too. Also get of unnecessary voltage sources - especially floating ones- in the circuit. It makes it much harder to read and isn't very realistic.
James