Nice project and TX sector is very fun to play. Regarding your ground mod......please remove the Ground connection to the heatsink from the +5V power supply. You are introducing a potential problem here...... Why? The LM338K regulator which is mounted on the heatsink has +12V at its case and is isolated from the heatsink by a very thin piece of material. When damaged there will be +12V at the heatsink and when connected to Ground there will be a blown fuse (and no +5V). The Ground wire you connected to the right lower pins at the power supply will do the job. No need to connect the heatsink as well. The above problem counts for other system 80 ground mod's as well. Most people use the +5V power supply as a central GND. It is fundamental wrong..... Use a hinge of from the lampboard or a screw left below in the backbox for later system80A games. I repair many Gottlieb games and I encountered this problem a few times already.
Marco