[Gunner]

My car has the narrow MSD distributor and the ERA-supplied expansion tank, which has a spacer of about 3/8 thickness where it bolts to the intake port. There is about a quarter inch between the lip of the tank and the nearest spark plug wire boot.

The only reason you should need more space is if you're using the wide MSD distributor, or if your intake has an unusually deep inset on the thermostat port.

If it's the former, my 'umble suggestion is ditching the fat-top for a standard configuration disto - the look will be more era-correct and the only real advantage to the wide distributors is reduced crossfire at very high RPMs. As FEs top out at about 6k, you're unlikely to have crossfire problems in any case.