ago. I can't recommend the M head, or maybe mine was shot after 40+ years of use (when I got it). The spindle was a bit too flexible for the stuff I was getting into. The worst, by far, was any attempt at boring left ripples in the surface. Part of that might have been the small shank I needed to use with the B&S #7 collets mine used.
So, I did put a wrecked J-head on it. It is MUCH too heavy, but Bridgeports are built tough, so nothing has snapped off from the weight, yet. My J head had a mounting plate like they use
to mount multiple heads on a T-ram machine. So, I machined a flat circle on the back of the casting so it could be bolted to the existing round-ram knuckle. When tramming the head you realize WHY later Bridgeports have worm screws on the tramming adjustments! It is REALLY hairy to do this all by yourself.
I had an end mill holder stuck in mine. I still don't know if it was the wrong taper or just got rusted in place. Massive hammering and prying didn't go anywhere. But, the machine was usable, as long as you wanted to use a 3/8" shank tool. So, I milled a pair of plates with C-shaped cuts in them, and drilled and tapped one to accept pusher bolts. I wedged those between the spindle nose and the flare on the end mill holder. I still had to ice the holder, heat the spindle with a torch, and beat on the drawbar to get it out. That isn't going to work for a collet, however. If some serious punding doesn't work, is there anything IN the collet right now? Depending on the size collet, you might be able to get a bolt up into the drawbar threads. If so, then a set of washers could be rigged to pull the collet while pushing against the spindle nose.
A couple other gotchas in a machine this old. The ways are not hardened. The original Bridgeport only had oil ports (Zerk fittings) on the right side of the X ways, so the far left end of the ways had to receive oil along almost 30" of oil groove. The oil never made it to the far end.
Also, some people greased these machines. Like ME! I had no idea you were supposed to oil a Zerk fitting. That probably didn't help the oil to get down there. I finally drilled into the saddle and installed a new set of fittings at the left end.
The leadscrew nuts are bronze, only slightly softer than steel. So, the leadscrews eventually get worn in the most-traveled regions, and the Y has to get what oil drips off the X screw. (The nuts are oiled through a little setscrew in the middle of the table. The table is put in the right place by aligning the "OIL" marks. Once the leadscrews are worn, you can't get screw holes to line up in mating parts, it is maddening, due to the differential wear in the screws. You can also get a concave wear pattern in the underside of the table, so the table can rock when you reverse X travel directions. You can also get looseness in the knee ways causing the knee to rock front/back when you raise/lower it.
So, you should just be aware of some of the wear problem areas in a machine this old.