Presumably then this won't be going on the road or used where it needs to be DoT compliant for transportation.
You should have a large enough opening to be able to enter the tank, in the top. Generally such tanks will have a hatch, which may be locked or boltled, but usually secured with a latch, during filling.
If the tank isn't rusted inside, then flushing the tank to ensure it's free of unwanted material is a good idea. If it held autofuel and has been empty, it may be okay. If it held autofuel which sat in the tank for a time, you'll need to replace seals and hoses, and pay close attention to any gum or sludge in the tank.
The best cleaning for a tank is a steam bath, but if you don't have a way to quickly dry the tank, or if it's an older tank that already has oxidized inside or isn't finished, and there's no way to air it out, you may be doing more harm than good and causing rust to form.
Make sure your pump, tank, and mounting structure are thoroughly bonded, electrically.
Check local regulation for requirements on spill barriers or containment. Many jurisdictions have regulations regarding single wall containers, and may require a barrier or containment under the vehicle if it's mounted on a truck. Ensure you have a fire extinguisher mounted near the emergency shutoff, or shutoff valves. If you're buying through a fuel dealership, especially a brand dealership (eg, Chevron, etc) vs a jobber or supplier, often the supplier will have good counsel on the best way to set up your fuel storage. They can also often be a good supplier of parts, including seals, gaskets, nozzles, etc.