Sounding good! Did you remember to level the base before installation?
Most shop cranes are rated at only 1/2 imperial ton at full extension, or 1000lb, and the mill is a fair bit more than that (the "specs" say 1250 lbs or so). I have a crane available that's rated at 1 ton full extension, but even with that I was uncomfortable lifting at full extension even a few inches off the ground, so I can imagine the thrill ride you must have went through! If you have to do it again you might try a couple tricks that I've seen used when forced to lift heavy loads with cranes:
1. Re-inforce the inside channel (the one that extends outward) of your lift with either another smaller piece of channel, or we used to use 2 pieces of angle iron welded together like a Z so that it would slide in easily with about 1/4" clearance.
2. Use chains or a come-along on the top of the vertical "beam" at the back down to a chain wrapped between the rear of the 2 legs, helping gird up the 2 small arms that extend down there.
Those 2 modifications would allow us to lift substantially more weight, but at greater risk, of course. The biggest problems then become your jack not having enough power to lift the weight (I've seen one give out at 1 ton over weight limit - frightening to say the least), and that when you're lifting it very high the whole unit becomes unstable as everything flexes, and you risk toppling.
I don't even try those types of lifts anymore, too heart-stopping for me. Gantry cranes are much nicer to use in those situations. But, of course, high limits can be the problem then.