This is a modification/upgrade I'm doing in parallel with to the alternator mounting bracket build. This is by no means required, most people just use generic parts store "slotted bar" style tensioners when they replace their alternator. I wanted something a bit heavier duty but with a lot of adjustment.
From my R/C Helicopter hobby I'm familiar with 'turnbuckle' style servo linkage adjusters and I've seen this style of belt tensioner on engines before. This is also the same basic concept as your steering rod ends.
The parts list is all McMaster-Carr, although I'm sure you could source these same parts from other places. McMaster is just convenient and has a very good selection. I'm using ball joint style rod ends to account for any minor mis-alignment between the head bolt and the alternator bolt.
Starting with the head side, the head bolt is 3/8-16 so we need a '3/8 ball joint rod end'. I used part number 60645K34, which is a Left-Hand Internal Thread version.
From there I used a section of Left-Hand Thread 3/8-24 threaded rod, this matches the internal threads of the rod end. Part number 95625A305.
The coupler in the middle is 2-in overall length 'Turnbuckle-Style' connecting rod. It has 3/8-24 Left-hand internal threads on one end (the side denoted by the groove cut into the aluminum body) and 3/8-24 Right-Hand threads on the other end. Part number 8419K13.
Next is a section of 3/8-24 Right-Hand threaded rod, part number 98957A138.
Last is the alternator bolt rod end. The Alternator bolt is 5/16-18, so I needed a '5/16 ball joint rod end' and it needed to have Right-Hand internal threads to make this work. Part number here is 60645K92.
You will also need jam nuts at each threaded connection so that this setup doesn't vibrate out of adjustment. For the left-hand thread I used Grade 8 Yellow-Zinc Chromate 3/8-24 Left-Hand Thread nuts, part number 99891A125. This is a pack of 25, you only need two, but this was the smallest quantity I could order.
On the right-hand thread side I used part number 94895A815. Again, these are Grade 8 Yellow-Zinc Chromate 3/8-24 Right-hand Thread nuts.
You really don't need Grade 8 or Yellow-Zinc Chromate, but the Grade 8 means that the nuts won't be the weak-link here and the Yellow-Zinc Chromate finish means that the nuts will have some corrosion resistance.
From here, just figure out how short to cut the threaded rod and assemble. You should cut the rods just short enough such that when the adjuster is at it's shortest adjustment you can get a new belt on. This will take some playing with lengths to get right.