Vacuum on 1968 GTS 340 4 spd.

The heads don't 'require' adjustable rockers per se, but it makes adjusting everything a lot easier. Otherwise, you may well be changing out pushrods.

You can do an approximate check on cam timing by:
- Find TDC on #1 by using a piston stop; don't count on the damper 0 mark as it can move/slip. By using the piston stop, you will be able to verify the damper 0 mark to be sure your ignition timing is actually what you think
- Once you find true TDC on #1, remove the driver's side valve cover and look at the position of the intake and exhaust rockers on #1. If both are fully closed, then rotate the engine 1 full turn to get back to TDC, and look. The 2 rockers ought to be open a slight and almost equal amounts. The intake might be a bit more open.
- One thing that can throw this off is if either lifter on #1 leaks off pressure when sitting. To avoid, this, run the engine a minute or so before doing this test.