This stock headlight is rated 35w low/ 55w high.
Is there a sealed beam 55w/60w light that will be plug and play with the existing bucket?
This can be a little dangerous. If the wiring is designed for 35/60W, you're adding a lot more to the 35W circuit than was intended. Chances are nothing will happen and it will be fine, but there always a chance that wires could heat up, melt, or even set on fire. The switches on these are quite simple, there's no relay and all current goes through the switch which can also be a weak point.
I would recommend going for an LED bulb. They don't all look like spiders from the future, and you can get bulbs that just plug into the existing bucket. LEDs draw less current for the same amount of light. LEDs also have a significant advantage on old bikes. Whereas incandescent and halogen bulbs are dependent on voltage, which after 40 years on old wires is likely to have dropped significantly, LED headlights are dependent on current which is controlled internally, so a normal voltage drop won't result in a significant drop in power like you see with incandescent and halogen bulbs.