Well, I guess I'll have to answer this now that I've built one!
According to Kubota documentation, the LA 403 Front End Loader (FEL) weighs 800 pounds; the 60 bucket weighs 216 lbs empty for a combined weight of just over 1000 pounds. This dolly is designed and built to carry a 1320 pound load. All dimensions below are in inches.
Materials List:
2 - 2 by 6 by 52 top rails (finished dimensions: 1.5 by 5.5 by 52)
3 - 5/4 by 6 by 43.5 bottom cross members (finished dimensions: 1 by 5.5 by 43.5)
1 - 5/4 by 2 by 43.5 top bumper (finished dimensions: 1 by 2 by 43.5)
6 - 3 inch hard rubber castor wheels rated for 220 pounds SWL each
24 - 5/16 by 3 (#16) carriage bolts with lock washers and nuts
2 - 5/16 by 4 (#16) carriage bolts with lock washers and nuts
4 - 5/16 flat washers
The dolly is basically a 43.5 by 52 rectangle. All joints are square and flush. The 2 by 6 by 52 top rails are the surface the FEL rests on. The 5/4 by 6 by 43.5 bottom cross members are placed one at each end, and one 24 (outside edge to edge dimension) from the front end of the dolly to help support the bucket. The caster wheels are mounted to the outer bottom ends of the cross members with four bolts each and are bolted through the cross members and the top rails. This yields a top rail height of about 6 off the floor.
The 5/4 by 2 by 43.5 bumper crosses over on top of the top rails at the back end of the dolly to prevent the FEL stand arms from moving aft on the dolly while rolling the bucket up/down while connecting/disconnecting the FEL. It is held in place with the two 5/16 by 4 carriage bolts that go through the bumper, the rails, and the rear cross member to hold on the two rear caster wheels (along with the another six 5/16 by 3 bolts).
In addition, the last 2 of the 5/16 by 3 carriage bolts, together with two flat washers and a nut each, are placed up through the rails at the back end to bolt the FEL stands feet to the rail to keep the FEL from moving once it is disconnected. The bolts are placed on 38 centers, off set 1 inch to the right of the center line of the rails (to match the stands offset), and 3+3/8 from the back end of the dolly (1+3/8 from the forward edge of the bumper).
YMMV, N. Fowler