EFI = better driveability and better economy (depending how you drive of course!)
Side draughts = Sweet noise and simple to fit
In my opinion if driven everyday I would go EFI, if only a weekender then either way.
Carbs onto 18RGEU - no problems that I know of...
Depending on the series of the older engine it will have a higher compression ratio and better cam profiles compared to the 18RGEU, hence more power potential.
The W55 box should fit easily size wise, not sure about the rear mount lining up though. Someone else might be able to answer this. At a guess I don't think it would be that far different to the W50. Just becareful to get the same shifter position on the box.
The coil from the 18R will work on the 18RG points distributor. If you have electronic ignition then you will need a different coil for the ignition system. The 18RG and 18R distributors have different advance curves so if possible stick with the 18RG distributor.