To answer some of your questions.....
The transmission will need to be replaced definately. Will the driveshaft be able to handle a V8's heavy instantaneous torque? Will the axles break? The tranny is replaced with a tremec 3550 tranny as using in ford cobras...made for over 350lbs of torque
Will it work with airconditioning or will all things in the engine bay have to be removed to make room for the bigger engine? Yes....We are stripping out all unesscary stuff...like abs pump....ac......
Will the ecu and electric system work or will all that have to be customized? Completely gutted out...it will be a carburetted engine.
Will the miata's small gas tank be reasonable for the V8's extra fuel needs? Plenty good..its' not gonna be made for road trips.
MSD your welcome to come over and drive it once my friend gets it finished
And he just bought a good used block for $50 we just have to send it in to get bored over and cleaned.