Was gonna reply to this thread yesterday, but didn't get the chance to. So:
Not only is it DOHC with four valves per cylinder, it'* also of a 'pentroof' design. Think hemi. But hemi can't do four valves per cylinder. That'* where pentroof comes in.
Above are two "G" type Toyota heads, aka performance twin-cam. The two-valver is the 5M-GE, same as ol' Cressy and the second-generation Celica Supra. The four-valver is that of the 7M-GE/7M-GTE, used in the 89-91 Cressida (NA only) and third-generation Supra.
5M (below) is hemi, 7M (top) is pentroof.
The "F" type head has valves at a narrower angle than the "G" heads for the sake of economy.
If you are going to pull the head for performance'* sake, be sure to have the bottom of the head machined (it'* aluminum) if you want the gasket to last; the block should be relatively free of any pits, as well. And have valve seals done, lose any potential current or future oil burning problems.
As for rear discs, you could try and look into ones from the GT-* coupe, but I'm not sure at all as to the interchangability.