No tensioner, just levering the alternator up the bracket it slides on, stretching the belt tight.
If case you need it, I used a Gates 11A 1195 on mine. That puts it about half way down the adjustment. From memory the GM number it calls for is about 1/2 to 1" longer and right at the upper end, no room to tighten and tall with hose running around it.
The alternator is the easy one of the lot on a 403, the rest have to come off if doing a belt further back. If you have a smog pump you may have to loosen it off and remove belt too, then put back on.
I'm assuming you have regular 403 in Firebird brackets, so only two bolt to touch. The short one that slides in the bracket, and the long one that it pivots on. Loosen only, don't overtighten into alloy, some have threadserts, some don't. The lone one at the base that goes through the power steering brackets and into the alloy bracket is often half stripped. If so, replace with a bolt a tough longer and add a locking flange nut on the rear near the valve cover.
No need to remove fan, feed the belt over the blades and then back to the pulleys, then redo it when you realise you want the writing to line up!
I use a longer flat bar, or tyre lever wrapped in rag to protect paint. Poke it down under the alternator from above the water neck down towards steering box, feed it through and onto one of the spacer tubes or bolts, or even power steering pump body. That gives you something to lever and essentially lift the alternator up and get a firm belt. Then nip the bolt/nuts up. If the engine running and revs make the belt squeal at all then tighten the belt a touch more.
Old post, a few pics may be useful, otherwise I have hundreds more. Once you look it's really simple.
https://forums.maxperformanceinc.com/forums/showthread.php?t=813887
