Hooking up a propane tank

Here is the deal.  Have a pretty big mostly full tank from the old motorhome.  It weighs about 100# and is as big as two barbeque bottles, so there is a decent amount of gas in it.  Has a regulator and what seems to be an NPT fitting on the end of the regulator.  In the garage I have a propane torpedo with a barbeque connect fitting.  Other end of the hose is what I think is an NPT thread.

Once I use up the gas I can get rid of the tank but it needs to be pretty much empty.  I don't want to spend a fortune on hooking this up but am not sure how to progress.  Tried the RV store but they are idiots.  Next stop is a gas grille shop but I have a feeling they will shy away from this due to liability concerns.

Anyone have a safe cheap idea?

Propane adapter. Old style to new style.

You probably need one of these. Remove regulator and install current style regulator. 

Here are the instructions

this should show you what you need.

Thanks but that won't work.  Regulator on tank is 40 years old and not moving anywhere.  Tried PB blaster but fittings are all brass and not moving.  I am a little concerned about breaking something and having a massive propane leak/explosion.  So the NPT on the regulator is my connection point.  Is NPT thread typical for propane connections?  An old part in the garage threaded in but not even sure about that.

Yes. NPT is a standard connection. Use a tef tape rated for gas. 

So try it with NPT, some Teflon tape and check it with soap bubbles.  Uh, no smoking and you should at least be able to tell if the fittings will work without blowing yourself up.

Thanks. Will be safe with this. Blowing myself and the garage up would ruin my whole day!

I really, really hate being on fire.

So the first RV store had nothing.  But the second one had a bushing to reduce the NPT thread size and an adapter that gave me the same connection as a barbeque bottle.  So with everything outside in case of explosion I taped the threads and tried it out.  Well with two regulators it seems flow is reduced but it does work.  Not getting as much hot air out of the torpedo as usual but it is working. so I can use up the tank and have free heat for a week or two. 

Thanks for all the info.