Currently the simplest way to convert a diesel tank to VO is to drain it, remove it from the vehicle, and install a "HotRod" probe type coolant heated fuel pickup. If possible you want to also clean its interior since paraffin from diesel fuel can create a dirty wax buildup inside old diesel tanks and this tends to loosen when exposed to warm VO. This will probably quickly clog on-board filters for several thousand miles so it is a good idea to buy a case of cheap/disposable clear outer shell filters and install one in the VO line on the tank side of the VO filter where it is easy to swap out with a clean disposable filter. It is also a very good idea to install a vacuum gauge so you can tell the filter is clogging before your engine simply starves for fuel and quits.
The HotRod How to file is available for purchase at: http://vegoilconversions.netfirms.com/newpage2.htm
Info on ordering a HotRod and 1"NPT weldable ports is at: http://www.websitetoolbox.com/tool/post/voconversionbasics/vpost?id=1958147&pid=18502701#post18502701
To install it you will need to have a 1" NPT port welded or brazed into the end of the tank as close to the tank bottom as possible. Bulkhead fittings can also be used if your tankis plastice. The HotRod will simply screw in to this.
I also suggest that you have a second 1" NPT port added a few inches to the side of the one for installing the HotRod so if you wish to later you can install an inexpensive tank preheater (for use only in very cold conditions). And finally...I suggest that a third 1" NPT port be installed in the lowest part of the tank so you can easily drain the tank if needed.
You will also want to seal the existing fuel tank outlet/pickup.
Tips for swapping out the disposable filters: Keep a box of latex gloves next to the case of disposable filters. If you pinch off the line on both sides of the disposable filter and grab it with a gloved hand to remove it you can then just remove the glove from your hand and the dirty filter is "bagged" inside the reversed glove. The wrist section can then be tied in a knot to seal the oil dripping disposable filter in for later disposal.
To clamp off the lines get two "needle nose" vise grip pliers and modify them by slipping a 2" section of fuel line over each metal jaw. This prevents them from damaging the fuel line being clamped off.