The hard part is getting all the weld cut. Welding a 3/8" hole isn't that much different than 5/16". Did try the Blair cutter, got 11 holes out of both cutters. Picked up a Princess Auto (Harbor Freight) cutter, same drill, same speed, got over 100 cuts form one cutter. Some bits work, some don't. Slow speed and a good pilot hole seem to help. Carbide cutters work OK, but can shatter. With the planets aligned and holding your mouth right, you can do a lot of cutting. If not, well...
I had to do some fairly extensive reconstruction on the cab of my '65 Ford flatbed... got the Blair 3/8" set. I was somewhat disappointed in the cutter life also, but it was a fair bit better than 5-6 holes per cutter! Maybe 20-25... but I think my one-speed air drill was a little faster than recommended... As the instructions explicitly state, keep it under 500 rpm and lubricate the cutter. Overspeeding will ruin them quickly due to excessive heat.
Interesting that the Harbor Fart set actually lasted that long. My town finally got a store this spring - I may buy a set and check it out
I agree that it's not hard to fill a 3/8" hole. Sometimes the 5/16" can actually be more difficult because it's hard to get it started and circling before the hole fills up!
