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Amazon is the best price I've found for the belts. I replace them 1st thing on the 4 (5th was a parts junker) 'wings I've had. BTWm Comstar (78-79) wheels have the speedo drive on the leftm spoked on the right. The FSM has good assembly drawings of each. Also, follow it when mounting/tightening the bolts there is a sequence that must be followed and clearances measured at the calipers.
Its on the randaks but you can keep the 2 halfs either side of the plenum together and not bugger the linkages etc. You can strip the bodies in the pairs and clean them etc. Saves a ton of fiddly work.