The most useful mod was putting a momentary switch on the floor to run the wipers. I can mist-wipe any time I want, and no fumbling for the dash switch.
http://forum.73-87chevytrucks.com/sm...?topic=27235.0
Hooking up an original thermac from a junkyard and cobbling up a heat muff and stack for the headers helped cold weather carburetion immensely.
The silver cold air inlet has since been replaced with an aftermarket black flexible one, and a chrome lid on the thermac. There's no fan or shroud because I went with an electric pusher fan.
Adding a second fuel gauge in the empty dash position so both tanks had their own gauge was cool.
Headlight relay kit from autolumination has worked well. About a fifth of the way down this page, they call it "automotive headlight brightener". I bought the "H4 9003 P43t" for my 78. $25. It came with the ceramic sockets, and the existing headlight socket plugs into the harness. I didn't have to change the harness they sent at all, just connected it to the battery, plugged it on to the headlights, and then plugged into one existing headlight socket.
http://www.superlumination.com/headlights.htm
I also put in a set of Hella glass headlight baskets that take the plug-in bulbs. Also under $50.