the 75 was an hugely popular rig when it came out. only reason you hear less of it is due to the release of the katana. everyone wants the latest and greatest right ?
the 75 was designed for what you want to do. it came from florida cave divers using steel tanks. so it will be great for using steels up your way.
as said previous, you only need to add drings and sliders etc to the waist belt for an attachment point for the allum. do not try to use the butt rails. unless you are planing on adding weight to the allum so they behave like steels.
contrary to what you will read online, adding weight to allums is not the end of the world. for ow diving in cooler waters it may actually be easier for you. if you were diving a minimalist system in warm water or caving then i would advise against it. but otherwise, if it works best for you then go for it.
you will also need to replace the stock bungee if you use allum. the bungee that usually comes with a 75 is too heavy. but that is easy.
all that said, i think you will prefer steels for cooler water diving. so this rig will do well.