I just ordered a Pit Bull spool insert kit and I wonder how far back from the axle I should drill and if anyone has any suggestion, Right now I am using end plate spools that work but they are a bit too big to fit into the swing arm.

 

My suggestion is don't. The pitbull insert kit looks like a pair of rivet nuts.
1. Rivet nuts don't work well on very thin metal - like the swingarm on an F2.
2. They are designed for pullout forces, not lateral forces like the swingarm stand will exert on them.
3. If they are behind the axle, they may block the axle adjuster blocks inside the swingarm. You want to still be able to adjust your chain.

I've see these used before. They will work themselves loose over time. My opinion is that they are not a good solution.

What I've done in the past that works fantastic is:
1. find a block of square scrap steel
2. cut 2 chunks of steel and round off 2 corners - so they look like D shaped blocks.
3. drill and tap some 6 or 8mm holes in them.
4. mig weld them (flat side) to the underside of the swingarm.
5. bolt on some spools and you're good to go.

Cheap, rock solid and don't get in the way of the adjusters.

 

Or do it the really easy way.
I just found these:
https://www.pit-bull.com/weld-on-spo...-929-f0019-000