Yea jus use washers or shims for the crank to driver.
The brass bushing that rides on the driver, between belt. Some don't need it but some do. It depends on the belt lenghts, belt manufacturers, quality of the belt. My grandsons buggy dont need it, with this comet belt. belt is not snug enough when at idle, plus it has 3100 rpm spring, so it's not grappy, jerky, or creeping at idle. And all my junk I have idling at 2000-2200 rpm. I like to get the oil up to temp and slung around as fast as I can. But if I started using a different brand belt, then it might need it.
Now mine and my son's mini, need the brass bushing, but we are running the exact same tav set up, belt, driven, the belts are a lil bit more snug, so they will creep at idle without the bushing.
Its something you will need to figure out, if it creeps, yes if not, no. But if for some reason you have to change to a different belt manufacturer, or length, then you will have to check again.
If theres any rust in the driver or driven where the belt rides, it will eat a belt up. I have saved SOME, by taking em apart and cleaning em up on a grinder wire wheel. If they are pitted, they will eat the belt up. The belt loves a nice smooth, glass like, place to ride on, and they live longer.
As for the driver springs, (stall). It depends on weight, gear, tire height, and riding terrain. But, ALL engines will benefit with more then 2200 stock stall. tavs and most clutches will have 2200 outta the box. You can add too much spring and slow your ouuta the hole pull down. Too much spring will blow passed peak torq in the rpm range, and put less torq to the tires, causing it to pull outta the hole slower.
3100 is a good place to start. 3000 might be better, 2900 might be better, 3200 might be better, jus gotta put 'er on the road and see which 1 leaves the longest black marks. Testing and tuning is fun to me, til builds get too much, then I let the younger guys test for me,

Now the hemi makes a tad bit more torque then the non hemi, because it has .5:1 more compression ratio then the non hemi. Same cam, same carb, same muffler, jus more CR, makes a lil more power. If all things being equal tires, weight, gear, max rpm, you will see a noticeable difference in the 2 engines. COMPRESSION RATIO, is king when it comes to torque to the tires. People can buy all those fancy, dancy, pretty parts in those stage 1&2 kits all they want too. But they ain't doing jack $hit, til they change that head gasket, to get the CR up. It's ez, cheap like $3-$8, and the biggest increase you will notice in only changing 1 part.
I always go to a 7" driven on a kart/buggy, it has a lower low gear,
for pulling then the 6". And will always pull more weight at same rpm then the 6". Space might limit you to a 6". Them yerf dog guys will know more about that.
I hope i helped, if not jus ask. I got some running to do today, I should be around later.