In my opinion. It is not an easy job. El1 is 6 solenoids. 2 height valves and 3 Switches.
not that it can’t be done. But it is less work to go with wireless air or in my case, micro-level. So most go that route.
About the level control, the Cadillac pumps are expensive. But you if you want a better idea, contact Keith Vasilakes (look on the Black list). He has built and does sell a kit that will replace that with better operation and simpler installation than EK1 would be. It isn't cheap, but it is good.
About the cruise, the manuals contain a diagnostic, but if the speedo is working, it is probably just a cracked hose. The only part that is difficult to replace is the switch in the directional. That requires that you remove the streering wheel and that takes a handful of special stuff that you can usually borrow at a storefront car parts place.
Microlevel was really a power level system replacement. On your 77 you would have to make a mount for the control face. But the rest is straight forward. Use his 4 solenoids, buy An air compressor. For air lines you have a line from the solenoids to the air bags. (2). And a line from the compressor to the solenoids.
took Keith and I a 1/2 day of messing around to install. We did it in a campground, and I remember it being cold. But I already had the harness ran, and the air compressor in place. We just had to install his controller, solenoids and height controllers at the air bags.
The wireless air(does not go over 100psi i believe). Was sold by applied gmc, and also jim Hupy I think made them up for Jim K.