1/8" HSS at the highest speed your Bridgeport is comfortable with should be fine. The last one I used was loud enough at 2000 RPM that I could no longer hear the end mill cut. I run 1/8" HSS at 3200 RPM on the Clausing. That's a skosh high but not excessive even for steel.
One advantage of milling vs bandsawing is that you don't have to square up the rhomboidal center piece afterwards.
If the ends are square I think I would set a vise stop to put the cut line just past the end of the jaws and then make four passes, both sides of both ends, so you only have to mill 1/8" deep. If they aren't square a rod work stop clamped in a tee slot and touching the work at its center should get you close enough to clean and square them up easily.
Since my mill doesn't have a DRO or enough quill travel to accommodate the fancy gizmos I start a tap straight in the drilled hole by guiding the tap shank with the untightened drill chuck jaws and turning the tap with one of these:
Once it's started straight I back off the quill and switch to another tap in a more convenient holder.
My well-used Clausing mill has a smooth and sensitive enough feel to use it as a manual tapper
You don't need to tap #4-40 the full 1/4" depth. If you drill part way through from the back at one size smaller than the OD of the tap threads you can hand-tap the holes by using the larger hole as a starting guide. The very shallow cut is enough to force the tap to run straight but not enough to break it. A hole slightly larger than the tap saves time but doesn't force alignment quite as well. Use a stub drill bit bottomed in the chuck so you can swap it with a jobbers length tap size drill without losing your depth stop position.
Some people like this method:
Which one of those I use varies with diameter and material. Sometimes I even power-tap.
jsw