Good Advice from Rusty! I'm no electrical whiz-bang, but the wires that make up the cluster lamp circuits are 20 gauge wires and should be able to carry, in the neighborhood, of 8 amps. The general likely sized those fuses to be on the safe side knowing that they were not pushing the circuit with only a few bulbs. That may be why a larger fuse has not caused any problems for you. Do what rusty says...put a 3 amp back in and see what that gets you.
Here is a recent thread of mine that will show all the fuse sizes and locations along with "our" conclusions regarding the SFE vs AGC styles....
https://67-72chevytrucks.com/vboard/...d.php?t=808513