I wish I could but I can't help you with your base sealing problem since I have never had that problem with mine. Unfortunately, most of the shops that specialize in these carbs and systems do charge big $$$ for them. Your best shot might be either a large swap meet, ebay, or other internet markets. If might one to search the parts for sale ads on sites like here, HAMB or other motorhead sites. It seems like these carbs always turn up at swap meets with prices from reasonable to way too much.

When's the next swap meet at Webster?