Disclaimer: I am not an expert!
With what you have, I would suggest the smallest clutch sprocket you can find, which I think will be 11T. With the 72T rear sprocket, that would give you a 6.545:1 ratio. Since both jackshaft sprockets are the same tooth count, the jackshaft doesn't affect the overall ratio. That
may work for you but I think that ratio
may be a bit low for your large OD tire. I would try it and if it doesn't work for you, you can alter the overall ratio by replacing one, or both, of the jackshaft sprockets. A larger jackshaft-in sprocket (which you may or may not have room/clearance for), or a smaller jackshaft-out sprocket, would give you a higher overall ratio (AKA: lower gearing). A smaller jackshaft-in sprocket, or a larger jackshaft-out sprocket (which you may or may not have room/clearance for), would give you a lower overall ratio (AKA: higher gearing). This may seem confusing (I'm having trouble just trying to make this understandable), but don't worry about the jackshaft for now, get a clutch and see how it works for you.
If the ratio doesn't work for you, then you can consider the jackshaft. There are jackshaft calculators (BMI Karts has one on their site) but it's just simple math.
If your jackshaft-out sprocket were 13T, your overall ratio would be 7.554:1 (with 11T clutch, 15T jackshaft-in & 72T rear)
The math
15/11=1.364
72/13=5.538
1.364x5.538=7.554 for a 7.554:1 overall ratio
You can easily plug in other numbers and do the math (or use BMI's calculator). Again, don't be concerned about the jackshaft for now, I just wanted to give an example of changing a jackshaft sprocket.
As far as tuning a clutch (springs, etc.), I can't advise you on that. If none of the experts on here help you, you could contact Jim at Max-Torque.
http://maxtorque.com/contact.php
https://www.bmikarts.com/go-kart-how-to-guides