That's a great way to get more life out of an old Pro! I'm more a tinkerer when it comes to clutching, but maybe can give you a couple ideas. To start with, I'd gear down as far as you can. I think a 46t bottom sprocket is the biggest you can get. I'm not sure if you can go smaller than a 19t on top, but that would help if it's possible. I know 19/46 is a good combination (will require a 74p chain); with a small motor like that, I'd look into a 18t or even 17t on top (can't guarantee anything on those, the FSM only references down to 19t). That will really help the engine pull that big track.
For the rest of it, it's very much an oddball combination, so you might just have to experiment. With the right helix and spring, you can probably get it to work ok without having to do anything with the primary. The helix that came with it is going to be too steep to work with that engine and clutch. All the RMKs came with a progressive helix, the idea being to allow the clutch to shift up quickly to get the track moving. With less than half the horsepower, you won't benefit from a progressive helix, and the final angle probably needs to be less too. Ideally, you'd find someone with a box of helixes (and springs) they're willing to borrow you. The shallowest helix listed in the FSM is Polaris part# 5135504, a straight 36º, which was apparently used on some of the old 340s before they went away. That would be where I'd start, if you can find one.
