Bill,
I installed the SK-337-40C kit on my 337C last summer and have at least three useable thermos bottle caps left over. They're worth $50 each.
rlhurt@gmail.com.
I bought the SK-337 kit for around $950 and it came with four new fuel doors, fuel caps, chains, fuel opening rings and all appropriate placards. Installation included Pro Sealing and riveting the adaptors in, riveting the doors on, and painting the doors. It took me about 8 hours and it does not require a licensed mechanic.
The main reason I upgraded the fuel caps is because I was tired of losing a gallon or so out of each aux tank any time I flew with them topped off.
Randy