The GM metric calipers are basically identical to the previous GM std. calipers, except for metric threads. One is 70's std. and the other 80's metric. Can't tell them apart, except for threads. I have no issue with either, as long as I know which one I have, and have used both.
If your Speedway axle is set up for '49-'54 drums/hubs, then swapping to anything else will mean changing to new spindles also. I have the Speedway axle with MII spindles and they are just spindles similar to what all axles use. No reason to be afraid of or avoid MII spindles and hubs, as they're as strong and well made as anything. Speedway axles came with either MII or early Ford spindles. The early Ford spindles have a huge flange for the backing plate to mount on, and I didn't care for the bulky look, so went MII. I wouldn't hesitate to go with the Speedway MII kit. Not much cheaper that works as well, and saves fabrication work. You can assemble parts and rework caliper brackets, etc., and maybe get it done for around $150, but it's more work and the savings is small.
Pretty sure the Speedway flat arms are as strong or stronger than stock, and look much better. I have them on both of my gassers and have used them on several other axles I've installed. They wont be a problem if you go that route. You may find that putting the caliper brackets forward will give you clearance and make it all fit with stock arms. Might need longer grade 8 bolts, and possibly a spacer to do so, but it can be done. I did this when I put a E100 I beam axle in my buddy's Nova and converted it to Cherokee rotors and GM calipers using stock arms.
Never heard anything about Speedway spindles being weak? Don't believe all the myths you read on the internet! I read all about their axles being thin walls and easy to bend. I had to cut and narrow the axle in my Austin gasser and was pleasantly surprised to see the wall thickness was nearly 7/16" thick! That's much thicker than any tube axle spec I've ever seen.
I've probably used about every rear suspension system imaginable over the decades. I have ladder bars with coilovers on my Austin. Comp Engineering ladder bars, and QA-1 adjustable coilovers. It works great, and launches straight/ Hooks up as good as I've ever had, and has so much adjustment that I never use once it was all set up. Easy to fabricate, and having factory ladder bars made fabrication pretty simple. They are short at around 36", but that works well on my 89" wheelbase Austin.
On my Falcon I have leaf springs with 54" ladder bars. Not using floaters, but instead I have shackle front mount to eliminate spring binding. I put new heavy duty multi leaf spring packs, and also have air shocks just for track use. They are plumbed with a dual line kit so I can add air separately. When I run on the street I put about 35 lbs. air in them. At the strip I run 50-60 psi in the right rear to keep the car launching straight with the 464 BBC and 4 speed combo. Works excellent, and it launches as straight as any car I've run, even wheels up launches.