I have a Sniper on mine. I got just the sniper unit and built my own fuel system, its pretty easy on these trucks and you can have an in-tank pump for pretty cheap.
Just a basic estimate I have about 2300 in mine but that includes everything I did, new plugs/wires, all the wiring stuff I did, timing control etc. You can do it cheaper if you dont want to include timing control, but Holley also has another distrubitor option available that is cheaper than the dual sync I bought.
Buy just the Sniper, not the master kit. For that additional 250 that the master kit costs you can pay for an in tank setup using all readily available factory parts. I have about 400 in my fuel system but I used all braided AN lines and fittings. You can use EFI rated rubber line and do it cheaper.
edit:
My sniper install starts here:
http://www.gmsquarebody.com/threads/78-gmc-forever-build.16379/page-16
more details and part numbers for fuel system stuff are all in there. I got about a 1mpg highway increase over my old edelbrock carb. Around town I think I might have picked up 2mpg. I think it averages about 15 now instead of 13. I also have an overdrive trans, and 3.73 gears with a 30" tall rear tire