How hard is it to install Rear Upper & lower Control Arms?

Not immpossible on the uppers the bolts to the frame are a real pain to get at . I drilled holes through the back seat panel to put a socket through , but my cars an 85 regal with a GN drivetrain so im not concerned with cutting stuff . The lowers are alot easie, do one at a time .I Just realized that you stated BMRs im not sure about those but the hardest part IMO is those top bolts.
damn, I hate drilling stuff.

I will crwal underneath and check it out.

Might just drop it off to a shop
You dont need to cut any holes in your floor. Use some common sense here. The bolts on some older cars can be a pain because they are rusted in there but just use some 6 point 18mm sockets and a couple of breaker bars and they will come out and a blast of PB Buster too. Before you go out and buy some tools you dont need try to bust them loose with a 1/2 drive ratchet and see if they are all loose. Here check out this link of my frame with the BMR arms and you can see how easy it is to get to the bolts. And I am talking about from underneath the car.
I have had a car where the bolts were so rusty it was incredible, I cut a small opening on each side of the center of the car with the back seat removed to get the bolts out because I had to pound them out the whole way. My other car (no winters) they came out no problem. If they are rusted into the sleeves of the bushings you could be in for a real battle. In that case sockets and breaker bars will not do it.

There was a guy on here who had the sleeve in a bushing spin rather than have the bolt break loose, search for that thread, he had a nightmare. You won't know until you try it.

Since you have new arms, cutting the old ones off would let you get the axle out of the way to hack the stubs out. this is the worst case scenario.

How rusty is your car?
Well Last time I was under there was a fuel pump install, saw some rust not to much.

I have no lift.

Hey I didnt say to start cutting , thats just what i did and it made it ALOT easier . Hey Eric its got to be easy to remove the uppers when the BODY IS OFF THE CAR ! COMEON ! Anyway BJMs idea about cutting them may help . Youll be able to do it but i bet youll shout a few bad words . It all depends how rusted they are to my car is a NY car from 85 go figure . Hey Fish you got yours off to easy so im sending a dumpster of snow and rock salt to Florida to dump on your car .
If they end up stuck, you might want to sawzall on each side of the bushings and just buy new bolts at any truck spring shop.I had to do that when I replaced the rear springs on my Suburban, the bolts were rusted in the sleves.
Tarey D.
Hey guys dont take my reply as an insult as that was not my intention. I know what you mean as I had the same problem when I parted out a 72 Skylark that spent its entire life in the salt belt. In that particular case I had to pull the rear with 2 of the arms still attached completely out of the car and hack them bolts out with a cut off tool. Some times soaking the bolts with PB Buster ahead of time helps. BTW the frame setting in my picture was made up of parts from 4 different cars and the 84 T-type I got the rear out of was the rustiest POS I saw and with the body still on all the bolts did come out with a liberal use of the BFH. Just put a 1/2 ratchet on the hex end and pound away on the point while turning it.