Can it be Done?
 

Is there a way to replace the gasket between the head and the intake tube on cylinder #5 on a '78 SC without taking off the whole fuel injection system?

If you can get at the rubber boot at the airbox and remove the clamps you can slide back the boot and possibly remove the runner but if one gasket is bad I'd remove the whole thing and replace them all. Take a look at one of the top post here a guy is having all sorts of problems because he may have dropped something down the intake and jammed up his engine. So be very careful about keeping track of all the hardware.