Natural gas line sizing with generator addition
Natural gas is conveniently 1000 btus per hour per cubic foot per hour so 320 cfh =320 mbtu/h. Your gas line with a 1.6" outside diameter is an 1-1/4" I.D. pipe. At 50' it will be capable of delivering 561 mbtu/h or 561cf/h

Here's a reference

Perfection pipe calculator

This chart is rated with a 6" water column and your 1/4 psi is 7" water column so 561 is slightly underrated from what you'll actually be getting. 

Edit

It seems the chart you're looking at is using a derated figure for natural gas' calorific value. 

From the link provided



From a google search



Most charts I've ever seen use the calorific value from the google sherch pictured above. 

I can only guess that that from your link they are going with the lowest quality refined gas available. I'm going to read the whole link later because I'm curious. It may be for "wet gas" but they are got rid of that stuff in the 70's through better refining processes.