Welcome to the world of 1964
I agree with the prior posters . . . your problem isn't likely to be the battery unless it has shorted cells . . . it shouldn't be causing problems with your Safari's wiring. My '64 Overlander has the same basic electrical system as your '64 Safari, and I am running three AGM Deep Cycle RV batteries in parallel . . . one of the three is shown in the photo below. On my coach, there were large fuses in each of the original battery cables (rated at 30 AMPS) . . . the installer that I worked with upgraded the cables (primarily because I was relocating my battery bank as there wasn't room for three batteries in the one-stop-service compartment). The new cables each had a 50-AMP in-line fuse. Since I also have three solar panels, my rig has a modern Inverter/Charger that handles electrical chores on the 12-volt DC side.
Some additional sources of potential issues on a '64 Airstream include any of the original accessories that had their own electric transformers. Originally, there were electrical transformers connected to the roof vent fans, furnace blower motor, and water pump motor. All of these devices had already been replaced in my coach, but I know of owners who have had headaches related to these transformers when the 12-volt system has been upgraded with new power converter/charger systems.
My suggestion would be to take your battery to a shop with the ability to do load testing, and determine whether the battery has shorted cells or some form of dead short that might be causing your problem. If the battery tests as good, I would suspect one of the transformers may be causing a problem . . . if any are still installed in your coach's wiring. Your coach's battery cables may also be an issue . . . I do know that when my coach's 12-volt was upgraded the size of the battery cable was increased considerably . . . I don't have my notes regarding the actual cable size, but it is much larger than what came with the coach originally.
Good luck with your investigation!
P.S.: You might want to check the production date on your Optima battery. We may just have an unconcerned dealer, but when we purchased three Optimas for our Sound/Show truck last month . . . two of the three were more than 10 months old and the newest was 30-days. We returned the two 10-month old batteries and insisted on replacement with two batteries with production date during the last 30 days. Optimas are expensive to begin with, and we were determined to have the newest of the new when paying $200 plus per battery.