I have the 'red key switch' (isolator) on the bulkhead on my '76.
Negative jump lead goes to engine mount. Positive on one terminal of the isolator switch - easier to reach than the starter.
Only have a problem on first start of the day if the car hasn't been used for a couple of days (or longer). Yes it's a poor connection somewhere in the main feed.
Next time I'm up to see a pal of mine with a ramp I'll clean all the connections...
Cheers,
