Zombie Slayer: New Fuel System

I haven't been around in a while, but I'm hoping to tear into my truck this year and get my M101 trailer bed on. With my bed off I'm going to be redoing all the fuel system. My rear tank has rust in it, so I've got new MTS tanks poly tanks. Also, the pickup was broken on both so I'm going to replace those as well.

My truck is still running rough. More of just a low frequency vibration. I'm wondering if the rust didn't clog up the injection system. I noticed the return lines on the passenger rear are leaking even though I only have a few years on them, and the truck starts right up even after sitting. I'm not sure when, if ever, the injection system has been changed.

Is there any way to verify that the injection system is causing the problem? I've heard about cracking the injection lines, but I don't understand how that helps diagnose a faulty injector. It does seem to be a little clackier than usual. I did run it a few quarts low but never lost oil pressure on my mechanical gauge, so I'm hoping I didn't mess anything up.

Anything else I should check? Compression? Do the harmonic balancers go out on these?

Thanks.

If you loosen an injector line, that cylinder will stop getting fuel, will stop firing. If that cylinder is actually getting fuel, actually firing, then loosening the line will cause the engine to miss, tightening it again will stop the miss.

If you can loosen and tighten the line without changing the sound, that cylinder isn't firing.

The rust could have gotten into your pump and/or injectors. Change the fuel filter ASAP and don't use the rear tank until you have replaced it.