I also have a danner mag 3 and they are both 1/2" pipe fittings. A 1/2 inch pipe fitting does not measure 1/2". I got male and female 1/2" pipe to 1/2" barb fittings at Home Depot and they work just fine.
I tested my danner mag3 when I got it by running it from one 5 gallon bucket to another through 2 chevette heater cores in parallel and about 4 feet of 1/2 I.D. Tubing and my flow was reduced to about 180 gph.