Just measure your wire. Fuse/breaker size should always be based on the gauge of the smallest wire on the circuit. Here's a pretty decent chart here to give you an idea:
If the top circuit is an 8 gauge wire, you can use a 40 amp. But if it's a 10 gauge wire, you should go no bigger than 30 amp.
But I wouldn't bother if it were my car. Pontiac built it this way. It's fine.
Also, 1969 service manual, page 12-37, figure 12-39 gives wiring diagrams and mentions of all the different option configurations for 69 Fbird. Just about every configuration has "circuit breaker" at that location, however a 69 with a power top ONLY had a "power take-off" and no circuit breaker at this location, and it feeds power thru to a relay. That "power take-off" is certainly referring to the non-fused shorting bar circuit that you have.
67 and 68 are wired completely differently with their power tops...no relay is involved on the earlier years, and the power doesn't even come thru the fuse box at all. But 69 has a completely different configuration.
Pontiac put that bar there and there is nothing wrong with it. Everyone seems to have forgotten that the entire electrical system is fully protected by fusible link(s), placed well before the fuse box. So if the wires coming into and out of the fusebox at this circuit is the same wire gauge as the main fusible-link protected circuit, then I can totally see why there is no need for a fuse at this location and why Pontiac put a shorting bar in place. If the circuit shorts, the fusible link will burn up to prevent a fire. So I see little reason to double-protect it as you propose. You'll just be protecting (sort-of) your fusible link with a breaker (assuming your fusible link(s) are unhacked). But you won't really be protecting it either, because as stated earlier it will just keep resetting itself and repeatedly putting power to the circuit, then cutting off, over and over again until the battery dies. So if you still REALLY think this bar needs to be swapped out, I recommend you put in a fuse instead of a breaker.
Why did Pontiac put a breaker in line here to protect the more heavily optioned birds? Just in case someone caught in a sudden downpour tries to run up the power top and 4 power windows all at the same time. Too much current is drawn, circuit overloads and trips the breaker, but no damage is done and nothing to fix...no fuse to replace...breaker resets itself and hopefully the second time the very wet and confused owner closes the top and windows individually next time. So those auto-reset breakers are really a convenience option more than a protection thing (auto-reset breaker is more convenient than a fuse), since Pontiac strangely decided to put a whole bunch of power drawing accessories at ONE circuit on the fusebox. But if a car has ONLY a power top and manual windows, there is no possibility of overdrawing that circuit with just the pump, and therefore no need for a convenience breaker. So Pontiac decides they might as well just put in a shorting bar instead. After-all, if a TRUE short develops in a wire somewhere, the fusible link at the battery or junction box (the horn relay on 69's I think?) will protect the car's wiring.
I obviously can't speak with any confidence to Larryneworleans fire, as I did not see the car...but I will speculate: It probably was not the direct result of the shorting bar being in place, but more likely the result of someone bypassing or eliminating or oversizing a fusible link at some time in the car's history. This has always been EXTREMELY common practice among ignorant shade-tree mechanics, and very dangerous because it usually masks an unrepaired short, and later causes fires. But if a proper fusible link was in place, it would have sacrificed itself and cut power long before any fire could start.
