Happy Easter!
Agree with all the above.
Might be easier to get the engine and gearbox done by the same people and in the same place - just a thought - that way if there's a problem with either it's down to one outfit.
Also it'd be handy if they have an engine and a gearbox dyno (although few places seem to have the gearbox dyno) so they can run them up before they go in the car.
Have heard good reports of this place but never used them
https://www.stubbsracingengines.co.uk/
Probably worth going to have a look at a few places and see who impresses you /who you get on with.
I'd get on with it as these things seem to have a long lead time in my experience.
You have many cars/restorations on the go at the same time - do you have plan or just trying to do them all at the same time?
Mine are just sat idle at the moment - waiting for minor parts from the States that are taking an age to turn up.
Cheers
Andrew