propane torch conversion to natural gas?

One problem you'll be facing is line pressure: have you tested the flow rate of your NG supply to insure an adequate level of pressure?

Here in the US, normally the NG lines are regulated to only supply 15 ounce/square inch pressure and, with the help of the gas company, there seems to be an upper limit of ~20 PSI. Will that be sufficient for your purposes?

No. I will have to add a compressor anyway, so any pressure within reason can be made available.