is that your propane for ht also?I think I burned ~80 gal in the shop last year. I put a 80k hanging heater. I went with the fan blown type heater cuz I figured it would warm it the quickest for short periods like in the evening after work. I did leave it on a couple of overnights when it was super cold and I had to be in there for sure. If I was heating full time, I'd probably do them tube style radiant heaters on the ceiling.
Have you chamfered all the holes? I've heard the holes can cause stress risers during ht if they are not chamfered. Blanks look good!