Cyclone is a good upgrade. Truthfully I think the best derailleurs are the older Shimano mountain units. The Deore/LX seem to be the workhorse. I think the pivots and cages are nicely made and hold up great.

Its time to upgrade the old derailleurs. For the 77 I'd recommend Cyclone or VX. For non period Shimano MTB parts work great. The cranks are awesome. Not too heavy but not light.