27 splines on the pinion shaft and 26 splines on the axles.
Yes, those gears would work.
Some people have got away with around 400 HP through the 7.5 rear end, but others have broke them with stock 305s, so you can't actually put a HP number on where it will break. The way it's set up, how it's used, what kind of tires are on it and a few other things can affect how it holds up. Any wheel hop can break it. If the gears and bearings are not set up correctly it will be weaker. Sticky tires will put more stress on it.
Here is something to consider. In '90 the 7.5 was upgraded with 28 spline axles. There are many good 28 spline differentials available for this rear end. This is a good upgrade for any 26 spline 7.5 rear end. There is a good chance that your axles are worn where the bearings run on them and they will need to be replaced. If this is the case then consider upgrading to 28 spline axles.
Another popular upgrade to the 7.5 is the T/A Performance aluminum cover/girdle. This cover braces the bearing caps and adds some strength to the rear end.
I'm not completely sold on the solid pinion spacer, but many people think that it is a must have for the 7.5 rear end. I have one on the shelf that has been sitting there for years and I haven't used it yet. I haven't had any problems so far with crush spacers.