Hi Herbert,
Joel's advice is, as always, spot on for a professional fuel polishing service and a tank fabricator/repairer to install access ports.
Here's a product I came across as an alternative, should you want to go DIY. Seabuilt Access Plate System.
Looks like a great design with a folding inner plate to make installation very easy.
I've got it on my "in-a-while-pile".
Craig SN68