Here's a better one?
Here is a set of instructions that doesn't seem to need you to drill and tap your fitting. Not sure about getting all the parts, but it sounds safer.
http://morgan38.org/discus/messages/1/13490.html?1116597305
Click on the photo to enlarge to see detail.
From the article:
"Parts list for CNG Adapter
1) brass nut (LH thread) – CGA 350 *
2) Brass nipple – CGA 350 *
3) “hydraulic Tee” high pressure** 1/4 inch
4) gage 0-4000 psi (Note: must be “no oil” type. Oil filled gages may explode!
5) Sherex/OPW LE-36 (3600 psi) receptacle ***
6) Adapter – joins Sherex /OPW to “Tee” ****. The “thread” on the Sherex end must be 9/16” straight thread.
Note: all fittings must be rated at 5000 (preferably 10,000) psi working pressure or higher. More is better.
* “CGA 350” denotes “CNG” fittings. Oxygen and other gases have a different CGA number. The fittings for different gases use different threads on their connections. Purchase these fittings and the gage from a welding supply shop. Total cost = $20.00. Most shops don’t stock CGA 350 fittings and will have to order them for you.
** High pressure Tee can be purchased at a hydraulic marine supply or repair shop. Cost = $8.00.
*** Sherex adapter and receptacle assembly is purchased from the OPW Corp. Total cost = $90. Order by calling 513-870-3100. It is a quick disconnect, ball lock type fitting.
**** Adapter can also be purchased from a Marine hydraulic. The problem of refilling these tanks using any CNG vehicle refueling “pump” is the fact that CNG powered vehicles are designed with SAE straight thread fittings. Our tanks are designed with NPT pipe thread fittings. This adapter (actually, the Tee) joins these two different thread styles together."