Get an L2 G3 accelerator.
That's about the only help you're going to get for your 6500.
Given that you've got a 6500/225, I you could probably get to 250 MHz without too much trouble. But anything more than that would probably be a crapshoot. I doubt 50 MHz will be possible, and I'd bet good money that 75 MHz will be a non-starter.
The 603e supports up to a 6x bus multiplier giving you 300 MHz on a 50 MHz bus. That also happens to be the same speed as a production model 6500, so there's not much point in overclocking yours when you could just buy a 300 MHz board. They cost about $20 - $30 if you are patient. If you are still considering overclocking, the only other thing you can do is increase the bus speed by swapping oscillators.
You could conceivably get a 55 MHz bus giving you up to a 330 MHz 6500, but anything more than that would be severly pushing the Alchemy board's various components that were designed to work at 50 MHz.
Go here for ideas:
http://www.bekkoame.ne.jp/~t-imai/pm55e1.html
I've never been one to poo-poo anyone wanting to overclock his/her Mac, but you do have to be realistic about it. Overclocking a 6500 just isn't practical when an L2 G3 accelerator is so much faster and relatively inexpensive. Also, the 6500 has poor cooling, so overclocking it may not be a smart idea. Finally, you'll only get a few MHz out of the deal, so it's not really much of a bargain if you are already struggling with CPU intensive tasks.
So in short, just get a new CPU. Don't bother overclocking yours.
Peace,
Drew