I have one machine with a HC on it. A Singer 99K. I named him Rusty. It works quite well. The machine is heavy enough that it stays put as you use both hands to sew with.
The HC actually came on a 127 but I never used it there, just took it off and put it on Rusty. The 127 will go back in a treadle cabinet as soon as I can get it redone.
3/4 or full size doesn't matter to me as long as the machine is heavy enough to stay put. A light weight machine can slide all over the place as you crank it. I don't want that.
Joe