If it helps, my parents old panel van will rev extremely high for about
5 minutes until it is warm.  Once warm there is absolutely no problem,
this began happening directly after a service where the idle speed and
accelerator cables were tweaked.  I'd suggest having a quick look at
the throttle with the car off and make sure it is moving freely, not
just the cable though. some oil on the linkages may fix this.
It seems possible that you have a leak in the intake system, after the
throttle valves.  Maybe the motion caused by dropping the car into gear
was enough to move the carb enough to temporarily block a leak?  I'm
not sure how you would check, maybe if you plug the intake before the
air filter or just after and see what happens, the car should stall
instantly