I’ve made multiple cycle truck forks. The challenge is getting long enough steerers. The first one that I made used a Bike Friday Tandem fork and extending the steerer with a long tapered sleeve. Subsequent ones used 1.125” x 058 (or 065) tubing with an 095 insert pressed into the bottom to make a butter section. It’s a heavy ass steerer, but stout, and that was built into a crowned fork.
The nice thing about 406 forks is that the lever arm is quite short, so you get a long of bonus strength just using components made for 700c forks.