When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
You need a small bellhousing C6 or a C4 for your I-6...check the classifieds here, on fordification.com, and your local online sites like Craigslist. Also check local pick-n-pull yards. Both are out there, you just have to look for a while to find what you want/need. If your tank is that rusted, I would pull it and trash it. Replace it with a better tank from a pick-n-pull yard...some folks don't like the in-cab tank, so they pull it and install a mustang or other tank between the frame rails back where the spare tire goes. That requires some modification however.
The 300 uses the same bellhousing as the smallblock V-8 (302/351W). Check for a C-6 in vans also. Is your C-6 in good shape or does it need rebuilt? You could find a good small block C-6 case and swap your FE internals for the small block case.