HEI Upgrade for 71 C/20?

[jrc55]

I've got a nice straight '71 C/20 (Camper Special) with a 402BB that I inherited a few years back that I use mostly for light street use and the occasional lightweight hauling (very handy). Aside from already having fixed just about everything else on this vehicle, the block and carburetor have been rebuilt and the stock distributor upgraded with a Pertronix Ignitor, but it still doesn't seem like it's combusting as good as it should and the mileage is awful ( <10 ). It's been suggested that if I upgrade to an HEI ignition system, I'll see improvement. Now, I'm pretty handy with tools, wiring, and simple tune-ups, but I'm not a "gearhead" and not knowing anyone else with one of these I've got a few questions that I'm hoping someone would answer for me...

1. Has anyone else done this to this particular engine and what were the results and pitfalls to watch out for?
2. Will the bigger HEI distributor fit on the back of the block that close to the firewall? It's kinda of tight back there. I'd hate the have that problem after buying parts.
3. What else gets altered besides the distributor and tossing out the coil? Plugs? Gap? Ignition wires? More?
4. Exactly what do I need to go out and get so I can do this myself? Junkyard HEI? New stock? New aftermarket? Suggestions?

Any helpful comments or advice greatly appreciated.

Thanks in advance,
Jim Chapman
Portland, Ore.