Mmm, you could do that, but why? The SPU-GT was produced in the days when MC cartridges were a rarity and matching step-up transformers even more so. I had one once and it was... just OK. You can do better. Any of the recent SPU cartridges, even the entry-level models, will blow the SPU-GT away. Pick up a good used step-up transformer or head amp, as long as it matches input sensitivity with whichever SPU you select. Trust me, you will be much happier. You don't need to spend $$$ on a step-up or head amp. The latter can be very affordable.
You didn't mention which model McIntosh preamp you are using. I'm not a mac guru but as I recall all of the preamps before the C47 (?) were MM only.
Also remember that you need to mate an SPU cartridge with an appropriate tonearm. The SPUs feature very low compliance so you will need a heavy, stiff tonearm to enjoy the best performance from these wonderful cartridges can produce. Good luck