A 3" system will give the most power as any neckdown from the collector flange aft is a restriction but a 3" system needs extra materials to keep it quiet enough such as regular type muffs up front then resonators (glasspacks) alongside the leaf springs for further quieting which allows you to run a less restricted (louder) muff upfront & still have an overall quiet system tho a 3" system will always be loud. Mandrel bends always. absolute best is restricted (quiet) muffs and a cable or electric cutouts right at the collector. Only open when you need em (& are kicking a$$) for a 10 second blast then quiet in normal operation. Your 85 (OE) will have a 4 pin ECU and a 2 terminal (single) ballast which will work for you & your choice of ballast can vary depending on what coil you select. holler back with what you are going with. The plug in, is that the 2 wire zigzag pull apart pigtail at the dist? set your initial with the vac gauge method then shorten the slots to get 35 total (& total is that initial+ slot (mechanical) amount). toss the heavy spring with the elongated loop on one end & sub in a mr gasket or MP light spring & want to be a bit under the pinging point at WOT up thru the gears on your hottest/driest day and can sub in a heavier spring for one of those two but one of those two is a good start. THEN plug in the can & want to stay just under the pinging point in everyday driving under varying load (in hg) and RPM conditions and CCW with a 3/32" allen wrench in the nipple makes it come in later (less advance)
This is all good advice for your project. The truck should be a beast...I am assuming that you are using a manual gearbox, if it were an automatic I would recommend a smaller carburetor.
I would recommend you go with an earlier rebuilt distributor (pre '75)and modify it with an advance plate from Don Gould at FBO Systems
www.4secondsflat.com . It takes all the guesswork out of welding and filing the advance slots in the distributor. He sells the kit with the springs for very little money. While you are doing the distributor take it apart and install one of the adapters to the bottom of it so that you can run a GM HEI 4 pin module for ignition to replace the ancient Chrysler transistorized ignition you might have or a spendy aftermarket ignition you may not really need. I have installed one of the advance plates in a friends bracket Dart and I put the adapter on the bottom of my 440 race distributor, I just need an E-Coil and a one wire 12 volt hookup to make it work. Simple is better.
