Hi Nick,
Regarding ground connections, at the Al's suggestion, I connected grounds for the ECU to the grounding lug that is welded to the inside of the left rear fender located beneath the fuel accumulator mounting bracket (red arrow in the photo below):
There are 3 through bolts with nuts that secure the bracket to the fender. Remove the 3 bolts, then remove the bracket and the ground lug will be revealed.
I ran a main ground wire from the fender chassis ground lug over to the EFI power panel (yellow arrow):
The orange arrows point to the ECU ground connections (pins 15-19).
The red arrow points to the 14Point7 O2 Sensor Interface unit grounds (black and white wires).
Regarding vacuum manifold: I don't have a brake booster, but the vacuum source is toward the engine. So I would mount the check valve so the engine can pull a vacuum on the brake booster diaphragm then not let the vacuum in the booster drop when you step on the throttle and the engine intake manifold vacuum drops.