What i know about the '76 bronco 302 is that it has cast pistons, hydraulic flat-tappet lifters, and the standard heads used on all late 70's vehicles with 1.78 intake and 1.46 exhaust valves, which is the same as the E6 heads or E7 heads for that matter. The '76 heads themselves are poor, but are better than the '86's high swirl heads. If you have to choose between the two, i'd go with the '76's heads. As far as the block goes, it has a two piece rear main seal (likes to leak/blowout) but has a fairly thick casting compared to the thin-cast 80's blocks.
What vehicle are you putting this in?
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2005 Suzuki Hayabusa GSX1300-R
1980 Ford Thunderbird - 255 V8
ported heads, 5.0L ported stock headers, O.R. H-pipe and Flowmaster 2-chambers, dual roller timing chain
hi-po Mack Truck hood emblem 
1985 Mustang GT 5.0L T5, F-303, GT40p, headers, off-road h, flowmasters, MSD stuff, etc.
Sold 02/06/04 
1989 Mustang GT ET: 13.304@102.29 mph (5-24-03)
Sold - 1998 Mustang Cobra coupe, 1/4 mile - street tires: 13.843@103.41 (bone stock)
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