how hard is it to change my 85 tpi 305 over to serpintine pulleys?

 

To do it the factory way, it's quite a bit of work. The poly-V drive system placed the alternator and AC compressor on opposite sides from where yours are located. the rest of the accessories have different mountins as well, but the change of theAC will require new hoses, evacuation and recharge of the system, and a different clutch on the compressor. The alternator change will require a little rewiring. The PS pump hoses may reach in the new mounting position, maybe not.

Frankly, unless you're really cramped for space, there is little benefit to the change. If you REALLY need space, get a bracket. accessories, and harmonic balancer from an LT1 and install those. That will make some space.