[v8volvo]

Neat setup, looks like that motor is really tucked in there tight!

As ngoma said, the alt belt has to be way tighter than you think it should be. For the faulty glow plug threads, yeah, helicoil is really the only option, since plus-size glow plugs aren't available. If it were me, I would be cautious about it, maybe even think about yanking the head and having the job done by a machine shop, unless you tap the hole exactly straight you'll have issues with the glow plug's taper seal leaking compression out -- but if you're confident you can do a precise job, then it shouldn't be too bad. I would probably take the injector out of that hole, set the relevant cylinder to TDC compression, and have a vacuum cleaner running in that location the whole time while cutting the threads, to ensure that any metal pieces were removed.

Idle speed is as ngoma described basically, but there are more complicated factors involved too, I suspect if you're asking about it then maybe your situation is more tricky. In the absence of the pump's external appendages, the idle speed is set by the throttle lever stop screw ngoma mentioned. However, there are two other things that can render that screw unable to affect things the way you want it to. If your cold start thermostat has failed or the cold start linkage is not adjusted correctly, there's an additional ball stud on the cold start advance arm that bumps idle up a couple hundred RPM, and that could be holding the idle up when warm too (if tstat is failed) and preventing the throttle lever from returning to its regular stop screw. If the cold start thermostat is frozen, it will remain in the "cold" position (advance+high idle) at all times, quite a common malady. You can temporarily resolve it by releasing the cold start cable end to disable the CS mechanism, and permanently fix it by replacing the wax thermostat, Bosch part # 1 467 202 302.

Or alternatively (perhaps concurrently!), if the linkage rod from the throttle "spool" assembly (the rotating device that the throttle cable attaches to) is adjusted to the incorrect length, or the sliding ball stud that it attaches to on the throttle lever is set in the wrong position, then the low idle speed can end up being set by where the spool hits its own low stop, which could be in a place that prevents the throttle lever from hitting its proper low stop screw on the pump. This is very common, and again has the result of making the proper low idle stop screw on the IP useless for adjusting idle speed. Volvo has an elaborate procedure for setting up the linkage down to the nth degree, but the only really important thing is that the rod and ball stud be configured such that when backing off the throttle, the throttle *lever* reaches its stop screw on the pump *before* the throttle spool reaches its own stop, and ideally there should be a good couple mm of clearance remaining at the throttle spool's stop when everything is at rest. Then you also want to make sure that when the accelerator is floored, the spool reaches its full throttle stop right around the time that the throttle lever is about to touch its high idle stop screw. It's a kind of fiddly process of adjusting rod length, moving ball stud around, etc until you have got it right, but this is something that can actually significantly improve the motor's driveability if the adjustment was far off (which can sometimes prevent the engine from reachin full throttle!).

Does your truck have a manual or automatic trans in it? I'm aware that Land Rovers were some of the only other vehicles to use the ZF 4HP22 automatic that was used with the D24T, suspecting you may have taken advantage of that compatibility?