OEM ground points are covered here:
MK3 TEWD On-Line
The primary block and ECU ground points are at the intake manifold and the rear right hand side of the head (black wire with yellow stripe).
If you're looking to ground things further a ground strap run directly to the block and to the head from the battery would be good additions. I was surprised to find that my Supra didn't have any significant ground between the battery and block. If yours does I'd like to know.