Another thing to look for is making sure your pushrods are of the correct length. Using pushrods out of a '74-'77 302 will hang your valves open when used in '68-'73 or '78-up 302 because these pushrods are longer to make up for the additional deck height the '74-'77 blocks had.
Some cam manufacturers use a larger base circle (typically designed for the adjustable valvetrain) and some designed to be used with the tork-down rocker nuts. If you try to install a cam that requires adjustibility, chances are that the base circle is larger in dia. than stock and could hang valves open when tork-down as spec. requires. Are you using aftermarket lockable or locking nuts on the 5/16" studs? Because if you have the 5/16" studs, you're suppose to tork them down......

As far as the bolt-down rockers, they didn't appear in the 302 heads until '77. From '74-'77 they used the tork-down style 5/16" rocker arm nuts. And from '70-'74 it was hit and miss, some had 3/8 adjustable studs and some had the tork-down 5/16" studs