Jason, the info I posted came from several books. They all say basically you can draw an approximate comparison to a conventional bow's power by dividing the crossbow's draw weight by 3. On the newer crossbows, I'm not so sure that's applicable any more. On an old one like yours though.... That's why I wanted you to come talk to these crossbow experts.

I was hoping you could get some reliable information from them on whether the performance level of that old Foxfire is up to the task of using mechs. Does it generate 40 ft lbs of energy?

I guess you'll have to chrono it to find out for yourself. Until you're certain though, I'd recommend sticking to fixed heads like Slick Tricks.