2 dead giveaways you need a new harness, the firewall connectors and at the starter, also check over by the alternator and oil pressure sender.
For replacement harnesses I deal with Dave Barton, he has guaranteed used ones for sale at great prices.
He also has many other aftermarket performance parts.
Disconnect the battery and start removing all the stuff that's in the way. I remove the intake on all the cars I do this to, it is much easer and faster in the long run.
I cut the old harness where it went through the manifold, removed the manifold nuts, the bolt that holds the brace, distributor cap, hose to the AMM, flame trap, and all vacuum lines, be sure to label them. Looking down at the old harness label everything or make good notes.
Also be sure to plug the intake ports, I about had heart failure as a nut bonced off the gasket face and dissappeared. Fortunately it did not go in.
I pulled the old 55 amp alternator out as I had a fresh 80A rebuild on the shelf. The ignition harness was OK but failing so I replaced it also. While you are down in this area replace the accessory bushings, this will keep your alternator in alignment and reduce the squeal so common in older bricks.
With the old harness out of the way lay the new one in place. You will see that there is only one possible way to hook it up, the connectors are mosly different, and similar ones (idle switch and air motor) are different lengths, as you will see the connector to the idle motor can not possibly reach to the idle switch, so use common sense and your notes and all will work great.
While you are in there it is a good time to replace those hard to reach hoses to the heater core. You can try to save a buck if you want, but for hard to reach critical pieces like these I insist on Volvo parts. This is also a good time to check the engine mounts and clean the oil trap below the flame trap. Double check all connections and feed the injector and ground wires up through the gap between #2 and #3 runner of the manifold. Pull out the plugs you put in the intake ports, add a new gasket and replace the intake.
Volvo injectors are batch fire, which means they all fire at once, you can't get them mixed up, put the wires on the injector that is proper for the length of the wire.
Here is a shot of the ignition harness in place with the new alternator, note additional ground wire (yellow) on the alternator. Running the ignition harness is the same as the main one, just take notes of wire color and location and lay it out and hook it up. Take your time and double check everything.
Here is everything bolted up and ready to go, the small hose running along the radiator hose is the new vacuum hose to the ignition box, the old one was cracking and I didn't want to run a rubber hose under the engine. While you're in there it would be a good idea to replace all hoses, cheap insurance.