I have the eventual goal of replacing all three of my new-style (cogged) drive belts with the original style (OEM or repro).  Since I am in the process of replacing my radiator, I thought now would be a good time to start with at least one (all three are in good condition, American made so it's hard to toss them).  I thought I'd start with the alternator belt since I think that's the one that squeals every once in a while when I goose the throttle.  I ordered a standard length repro belt for early '76 out of a catalog and tonight when I went to install I remembered why I couldn't do that.  The valve covers I have to use with vortec heads don't allow the alternator to rotate fully towards the water pump pulley so there's not enough slack to slide the belt on.  When I was putting things together last year I had to have the parts store guy help me out by figuring stock length and then going the next size up.  Is that my only option here or is there a way I'm missing to get the belt on over all three pulleys?  The belt I have is GM 9433657, 53.5".  Looks like I could get 9433764, 56" but it's not cheap and hoping to not have to do that.  

Can you possibly loosen the valve cover, and move it enough to get the belt on? I think I have the same problem with one of the belts on the '74, but can't remember if it's the alternator, or A/C belt....

Thanks, Joel.  I think I'd have to move it about an inch for it to clear and I don't think the rockers will allow that.  I think I may send the belt back for the time being and go with the cogged one.  What are the issues with using new style cogged belts?  Do they sit in the pulley grooves too deep?  And then they don't get the correct contact pattern so they slip?  

The "cog" belts are fine...IF they have the correct cut on them. By cut I mean angle, where it fits down into the V groove of the pulleys. The top of the belt should be even with(or just slightly above) the top edge of the pulley groove when installed. If it sits down in the groove, or way above the groove, it is the wrong belt. Belts are made with different angle cuts, and thickness of the sidewalls, as are pulleys. Newer stuff has a shallower angle, compared to older stuff, so most of the newer belts will sit too far down in the pulleys, and they do not make full contact with the sides of the pulley grooves, so it's only using a very small portion of the belt surface...which is why they tend to squeal. A belt that sits too far down in the groove also changes the speed of that pulley, since the belt is running on a smaller diameter area of the pulley. Same is true of the opposite...and old style belt on newer pulley puts the belt too far up in the groove, and that will speed up the pulley. It's not a huge difference in speed, but a difference none the less.
Also....a lot of the newer made belts are "stretchy"....they don't have the same nylon corded construction, so they tend to "flop" while running. The flop doesn't hurt the belt, but it can create a new vibration ya never had before, and can be hard to figure out if ya don't know about it.
Bottom line..if you can get a belt that you can install without disassembling the engine, go for it, as long as it fits in the groove properly.
I have an engineering paper somewhere that explains the different cuts of belts, the different angles used, and why they make them like that...I'll prolly never find it, but I'll look....

On my 77 to put the alt belt on I need to loosen the 2 bolts that holds the alt bracket to the block, get the belt on the pulley, tighten the 2 bolts then adjust the alternator belt tension and it just barely clears the valve cover.

Another question on belts - which belts should route around the water pump and crank, and which should route to only one or the other, and why? I have a feeling that my AC belt on the 76 was routed wrong. I took it off a few years ago when I replaced the stamped steel valve covers with aluminum ones. (Belt was too short to clear covers) always wanted to replace belt for one that fits, but want to get one that fits proper routing. Need to check my AIM again. Thanks

My back belt goes over the water pump and the alternator. My middle belt goes over the water pump and the a/c compressor.  My front belt just goes to the power steering pump. 

Mine are the exact same as Gary.  My '76 does not have an A.I.R. pump - not sure how that belt is routed if there is one.  

Quanta Products

here are the belt configurations for 1977