The pulley is splined like you said, which would present a problem. Another problem would be figuring out the rear belt size to accommodate the new smaller pulley. Also the vari-drive linkage will have to be re-adjusted as well, but once you get used to it its not too hard to do that. I've done it on my MTD quite a few times. It should be possible for you to do it, it just might be a bit of work.
"This'll either wake you up or put you to sleep forever!"- Red Green "Whatever you do you should do right, even if it's something wrong." - Hank Hill - MTD Off-Road Build
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- Allis B-110 x2, 710 & 716 - Wheel Horse 655, 953 & 500 Special - Case 444 - Bolens Versamatic & 1668 - Cub 106 & 124 - Ford LGT 125 & 145 - White 1650 Yard Boss