Lean idle. Rich cruise.

You don't have to. I have no idea of what you're working with or your goals with this vehicle. People certainly have run locked timing and made it work for them. MP setup their race distributors (aka tach drive distributors) for easy starting and to maintain high rpm timing flat or very slightly advancing. (graphed here) They basically accepted idle timing as somewhat unstable because they didn't think it was that important - at least relative to the other two goals (easy starting and top rpm performance).

The reasons to put a curve in the distributor are twofold.
1. It will likely allow the engine to run richer and stronger at idle. The better it runs at idle rpms then the less time for the carb to clear up when throttle is opened.
2. The top end should continue advancing mechanically so as to avoid the retard that comes naturally with most electronic ignitions. (If running dual points then not needed.)
As illustrated in the two videos posted here. lets revisit timing

https://board.moparts.org/ubbthread...mallory-unilite-timing-issue.html#Post2488599
Here's a couple guys who have run Stock Eliminator commenting on the subject, GTX John and 92b. What used to be done with dual points no longer applies with electronics even for drag racing. lets revisit timing

If you keep with the locked timing, then the idle will have to be lean. Timing goes hand in hand with conditions in the combustion chamber. Increase the fuel load and it will be denser, higher fuel compression and need less lead time to be converted into pressure to push down the piston in time to do work on the crank. Also more exhaust gases going out the port (more fuel going in) so ought to help in keeping the exhaust moving away.

That doesn't mean it can't be made to work with locked timing on at least some setups. Lots of guys do. if you go that route,at least you'll be aware of how it may be effecting what you need to do with the carb.

A large timing lead at closed throttle can help increase manifold vacuum, which in turn helps the fuel gasify and distribution. There is a point where there is no more gain. With a stick car you can get away with a leaner mix than automatic. With an automatic the engine needs to have power to turn the pump, converter, and gears at idle.