You can get the BRM's accessory belt off and back on from above, this is a pretty easy alternator.
Remove the charge air tube, fish it out from under the fuel and coolant hoses.
I swing the fuel filter assembly up and out of the way (no need to remove any fuel lines). Three 10mm fasteners.
Pull the tensioner up into service position and slip a pin in the hole to hold it in place.
Take the belt off... honestly cannot remember if you need to remove the tensioner to get at the alternator bolts or not on the BRM, but if you do, pretty easy, just two bolts, then the two alternator bolts, slip it up, make sure the battery is disconnected, then remove he wires and holder on the back side of the alternator.
Really easy job.
I would only replace it with a Bosch unit, and with a new INA pulley if it does not already come with one (it should, though).