This...
Even the 292/508 sb purple shaft shouldn't NEED to idle at 1200 rpm. Most SB's should be able to idle in the 800-900 range without issue.
Timing first then worry about the other stuff. If you go out and turn the distributor counter-clockwise a bit and it picks up RPM, your initial timing setting isn't right.
Got a vacuum gauge. Hook it up, advance timing at idle, if it picks up vacuum, turn the idle down and do it again. Find the max vacuum point at a constant RPM then back off timing so it's 1" lower than max. If it still starts when hot, you've found your initial timing point.
At 1200 rpm you may have mechanical timing bleeding in and when the RPM drops that timing that was there goes away.