Since the OP has a '68 273 then the 318 should drop in unless the '68 273 is a very early '68' and has a small crank register hole perchance. The one thing to be aware of is that the hole in the back of the crankshaft where the torque converter snout fits in (crank register), is larger in the '68's and later.
Yes, there are different 318 pistons out there (truck vs car) but they are all low compression engines; the trucks are just lower. This makes them 'soft' in the lower RPM's ranges; i.e.; not much low end torque. It is very desirable from a performance standpoint to correct this issue for the 318's, so a custom build speaks more to that. The same really applies to 360's. The later Magnu versions of these engines fixed that low CR issue to some degree, so IMHO are more desirable from a performance standpoint.
For the wife, I think I'd think 318. You have a light car to start with, and something that takes a lot of throttle management may not make here happy.... but you know here likes and dislikes better than us. I have an acquaintance who went from a '68 273 to a 400+ CI stroker, and he says it ruined the car as a family 'go to the drive-in on Saturday night' easy driver car.
The car 'dieing' randomly is a much different matter than the engine....look at the electrical stuff and brakes first.
