Here's some materials/measurements off my home made land plane: (it's covered in snow and somewhat frozen down so measurements are as close as I could get at this time.)
Side Channels: 1/4" thick 2.5"x10" channel 48" long made from pieces I had on hand.
Top cross braces for hitch mount: 3/16" thick 3"x3" square tubing 60" long
Angled braces for bolting the cutting edges to: 3/16" thick 2.5" angle x 60" long
Cutting blades removed from box blade: 60" long
I can put a "weight tray" across the top of this plane but found it isn't really needed. the land plane itself it plenty heavy. But even with the weight tray in place and loaded with railroad tracks for weight, my older JD750 (20 HP) had no troubles pulling it around in 2 wheel drive.
If you're making one to go behind your B7510 you shouldn't have any troubles pulling a 5 footer around, or bigger if you're thinking of a 6 footer. They pull pretty easy as most of the material is spilling over the top edges of the blades.
My blades are angled about the same as bearskinners. Also, no engineering was done, just used materials that were laying around.
Here's a couple more pics showing those pieces. the 2nd pic shows the plane put together except the box blade cutting edges still need to be bolted into place.
View attachment 53207
View attachment 53208