I think the point they are making is that the MK4 Supra is the only factory spec cooler worth using. If your going to go for an aftermarket unit then go for a specifically designed Z31 cooler or for a universal one.
Unless you want to use the factory spec cooler then ignore the Supra advise.
The only MKIV owner I know who switched to an aftermarket setup bought a front-mount core. I'd imagine that's common. So yes if you find an aftermarket MKIV core and it fits and you can plumb it then it will work. Unless you've stumbled onto a one of a kind deal then it's going to be much more expensive than a MKIV OEM core.
wow!! dont buy that , i dount know if they are any different but i doubt it but there is like a ton of no name intercoolers for sale that look identical and also have similar dimensions for alot less, look around.
by the way there is somebody bidding on a spearco kit for our cars for like 710 wich i think is insane!, you can buy a brand name intercooler of your choic for like five hundred that will most likely be more efficient and probably outflow that unit and get somebody around your area to configure the piping and finish the custom one of kit for probably a hundred less than 700, but that is if you need all that intercooling( you most likely dont) unless youre running over 400hp ( the supra is only good till 400), still i just bought one and i think when i need something bigger im just going to buy another one and have my local shop weld and make me some custom endtanks.