Garage door opener replacement issues

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gfw

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My 25 year old chain drive unit died. A gear inside completely sheared in half so without an unavailable 25 year old replacement part, it's a write-off.

I was hoping to re-use the existing mounting brackets, and even the existing rail if possible. The existing rail is a single metal "T" shape, rather than the multi-piece hollow bars that come with the new kit. Being one piece, it's nice and solid.

I bought a new unit and discovered some things I didn't know. I can still return it though.
I had thought "hey, let's try a belt drive unit because they're quieter and it's directly under my living room"
So the first thing I learned is that a belt drive will absolutely require replacing the existing rail with the one from the kit. Ok, I could live with that, but...
The second thing I learned is that the rail from the kit is about 8 inches shorter than the existing rail. That would require me to move the mounting brackets in a way that would be surprisingly complex. Right now, the existing unit mounts directly below a joist. 8 inches will put it halfway between joists, so I'll have to make brackets that bridge between two joists *and* not block the power outlet that is right there! Of course this is doable, but it's just annoying.

So I'm thinking of returning the belt unit and buying a chain unit.

Ok, questions start here.
1. Is it possible my chain is a non-standard length? The distance from sprocket to sprocket is about 120 inches. The door is 7 feet 1 inch tall.
2. Are chains standard in terms of chain link size? In other words, could I reuse the existing chain on a new chain unit? I'd rather use a new chain.
3. I see chain extension kits that come with replacement rails. If the new chain is too short, is there a way to extend it to exactly the length of the old chain and not bother getting yet another rail I don't want to use?
4. The instructions with the new kit say "doors taller than 7' up to 8' tall require a rail extension kit". What do they mean? Do they mean the regular kit will work with doors between 7' and 8', or do they mean you need the extension for over 7', and the extension allows up to 8'? That's terribly poorly worded.
5. Is there something else you think I should do?

I can post pictures, but I don't think they're needed at this point.

Thanks!
 
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