I have had two of the Burke Number 4 milling machines. On the first one, I built a jackshaft (or countershaft) drive. This was fabricated out of structural steel (angle and plate) and was bolted to the existing Burke motor bracket. The original motor was 3 phase and in my youth, I did not know about 3 phase converters or much else. I abandoned the 3 phase gear motor at that time, and built the countershaft drive. I made it heavy to approximate the weight of the gear motor, since this would tension the drive belt. I eventually used a 1 HP single phase motor with drum reversing switch, and used the Burke cone pulley.
This drive worked fine, the only problems being that:
-using a junkyard pulley for the countershaft, it was not quite big enough in diameter to get a low enough countershaft speed. Hence, the lowest milling spindle speed was a bit on the high side
-weight of the assembled motor and countershaft was not quite heavy enough to get the belt tension to where the gear motor had gotten it. The result was on what, for the Burke number 4, was a heavy cut, the cone pulleys would slip on the belt.
I sold that number 4 Burke in favor of one from the local high school. Much less wear and it has the original power feed mechanism intact and the Burke vise as well. This machine has a single phase "Master" gear motor. It is a repulsion-induction type motor. To reverse it, it is the usual method of shifting the brush ring. However, by mounting the cutters properly and setting up work accordingly, this reversing of the motor does not need to happen.
What I have found is the weight of the Master gearmotor is good deal more than my previous mill's motor/countershaft drive. The result is the belt stays tensioned and the mill can take a bit more of a cut. I forget the shaft speed of the Master gearmotor, but it is a bit lower than my original motor/countershaft drive. It is slow enough at the milling arbor to allow my using 3" and 4" diameter milling cutters.
The reality is the Burke milling machine is a small, light duty machine. It is rare that I ever take it out of the slowest belt speed, since I mostly work with steel. In my opinion, going to a variable speed motor drive is probably overkill for this type milling machine.
When I built the first mill's countershaft drive, I used 3" x 3" x 3/8" angle iron for uprights to take ball bearing pillow (plummer to our UK brethren) blocks. I welded pads of 1/2" flatbar to the angle iron uprights to give enough meat for tapping for bolts to mount the pillow blocks. I welded a base frame out of 4" channel (laid flat) with tapped pads to mount a 56 frame motor. The whole countershaft weldment was then welded to a piece of scrap plate with holes drilled to bolt it to the existing Burke motor mount bracket. If I remember right, I used 1 1/4" diameter shafting for the countershaft. I was working with parts from the local industrial surplus store, and from a junkyard where they sold me the largest single groove vee pulley that they had ( 10" diameter, I believe). I bought a new Brazilian made "Kohlbach" 1 HP 1750 rpm motor (probably now absorbed into "WEG" motors), and a new drum type reversing switch. I thought I'd made the countershaft heavy enough & slow turning enough. It came close to what the Master gear motor does, but not quite. I was running the kind of vee belting made up as loose links (the name escapes me) rather than take the head of the mill apart to replace the belt.
With the Master single phase gearmotor drive on my present Burke mill, I usually run slitting saws or vee-groove milling cutters. Having the luxury of a good Bridgeport which came into my shop after the Burke mill, the Burke is relegated to jobs that I can't do on the Bridgeport. For running slitting saws and vee groove milling cutters, the Burke does fine.
I keep it running at its slowest speed since most jobs are in steel, and use light sulphur cutting oil, and the little mill slogs along.
With 20-20 hindsight and the passage of over 40 years since I junked the original 3 phase gear motor, the ideal setup in today's world would have been to use a VFD to supply the 3 phase gear motor. No going back to it, so I live with the single phase R-I gear motor.