Never seen 12s in an 87 but i did help a guy with a custom box that held 10 8"JL subs with 10 amps built into where the back seat should be. Only problem we had was on the first test it hit so hard it warped the trunk lid about 4 inches.
But to help you out with best posible sound for it( and this might sound strange but) what type of music do you primarily listen too. The reason i ask this is certant size speakers sound better for difrent types of music. 8s for rock country 10s for rock country(some high end 10s can be used for extream bass out put with a good box) 12s Rap metal techno 15s rap anything over a 15 is just showing off you have too much money lol.
Or if your dead set on 12s the recent line of low profile subs are realy nice for tight fits.