If you could barely move it, so does your motor even if this lil' thing's got about 60 something more horses than any human can generate.
What happens is the track almost locks in place because of the tension it's got into it. That doesn't mean the motor can't spin the drivers and skip a tooth or two in the process even if the track is almost "locked". 60 horse is still enough for this to happen, I think.
Bad pivot design. The stock swignarm swigns the rails/track too far towards the back. This is where pivot relocation bracket comes : the idea behind that is to create a more vertical path to get rid of track tightening.
No matter how much work you do to your engine, this problem is still gonna be eating 1/2 of your horses or more. Mine felt like a poor running 503 the day I had this exact problem...
What you'd probably need is just a new design of pivot relocation bracket for your parralel bar and swignarm since the 503 suspensions are different from the other ones.