Do you have the front clip off already? I don't think a few inches will help much. If you unbolt the cab enough to lift it a few inches you might as well take if off. One quart would be barely enough for the entire frame. I like to buy POR-15 in the pint containers and not have a partially used quart sitting.
If you have an engine hoist you can take the cab off by yourself. Take a look at this thread
http://67-72chevytrucks.com/vboard/s...d.php?t=346948 . I used a 4x4 and cut notches to hold the door sill. It is very easy to balance the cab on the 4x4 and lift it up high enough to move the frame out.