Anybody else got the squat?

[Kudzupatch]

I am just guessing, but with a single leaf spring that was not designed for use like this, I would expect you are getting horrible spring wrap under acceleration. I remember some of the GM cars with mono-leaf springs, and they were designed that way, giving fits when the motor was beefed up.

Sounds like a really bad idea and I think you really need some suspension upgrades to make this work!