Is it possible/ practice to replace the main bearing cap without removing the motor. At every turn I'm running into road blocks.
It can be done. You will need to support the crank as close to the rear cap as you can for fitting the new cap. IT IS VERY TIGHT WORK SPACE ESP IN THREE DIP PA(I)N!
In my OP you will have MUCH less frustration better turn out if you can pull the motor. The weight of the flywheel/transmission will be bending the crank down and unless you have all your duckies in a row I think what you are trying to do might not be worth the time.
I pulled it off, and now my crankshaft doesn't have any end play, and my magneto is gapped correctly. It would have been way easier to pull the motor.
Seven hours from "can it be done?" to "it is done!"?
You think you could pull, fix, and replace the engine any faster than that?
The big advantage, also sometimes the disadvantage, to pulling the engine out and going through things is that you may find more that needs to be done and make a bigger project than you want to take on. Sometimes it is nice to fix the immediate problem and enjoy the car for awhile, as long as it generally seems okay otherwise.
Years ago, some good friends of mine tightened all the main bearings in a T one evening (about half the night, actually) on a national club tour. It was worth it for them.
Drive carefully, and do enjoy it! W2