Good luck with that 6spd. I broke a few of them when i used to race lawnmowers back in the late 90’s. The Peerless 3spd’s (600 series?) with the sideways H pattern are bomb proof if you can find one with a full set of needle bearings.
I used to run a 6” pulley on the engine, and a 2-1/2” on top of a 3” on the trans. I rigged up a derailleur at the trans to switch between the two pulleys that was actuated by cable attached to a lever that got pulled when i pushed the clutch all the way to the floor. The derailleur would toggle between the two pulleys with each push of the clutch.
I copied the mechanism similar to that of a push button switch that latches down in the on position and releases up in the off position to hold the derailleur in its 2 positions.
I was running on a 1/3 mile clay oval track breaking 60+mph in the straights. Needing to downshift in the turns to keep the power up, 2nd gear of the 3spd trans was just too low.
The derailleur setup gave me a perfect ratio in between 2nd and 3rd, and it made “shifting” a breeze. Kick the clutch coming into the turns, and kick it again on the way out. I could keep both hands on the wheel, and believe me, i needed it. That track was rough, especially with no suspension.
A few crappy pics of it.