In general (even on today’s cars) I dislike donut spares. I think you are better off keeping the current steel wheel you have and put something close to the size it originally came with - I used something like a 195-70-R14 for a spare on a 70 XR7 in the past. Might be obvious, but use the same construction as the 4 you will have on the styled wheels (not sure if you mean to go to bias tires for “period correct” but radials should work better). That should work well enough if you ever need it.
Just my opinion - but I ran a donut spare after a blow-out on my newer Audi A5 this week and it ran like S&**! Couldn’t wait to get it back into the trunk & I’m even thinking about getting 1 full size wheel/tire to replace it in the future…