See if this is what you need. http://www.plumbingsupply.com/bulkhead.html
Maybe thread the tank, and screw the fitting in the tank without using the backup nut, add a nipple and connect to your pipe with rubber hose to absorb vibration and prevent cracking.
I have a three inch one like that one that I never used. I couldn't figure out a way to tighten the fitting. My tanks are polyethylene. I was going to cut a hole and then cover it with more polyethylene, but a local plastics welder said the plastic would eventually crack.
I got my fitting from Indelco Plastics and it was between $30 and $40. A lot less money than plumbingsupply.com.