According to what I see on the Disraeli Gears site the '67 Lark used 2 cables in a push-pull arrangement: Shimano Lark-W derailleur (1st style)

In that case I'd say stick with the Sprint (rebadged Huret Allvit). But since you don't care about originality and those Sprint derailleurs can sell for quite a bit on eBay to people who *do* care about originality I'd say sell it, buy something new and cheap that works better and pocket the difference: Shimano SIS RD TY18 Rear Mountain Road Bike Bicycle Derailleur 6 Speed MTB New | eBay