After much research into the 40 series plate, I do not recommend them. Only the asymmetric belts should run with a backplate. This is due to the driven pulley lateral movement being opposite the driver pulley lateral movement. They would skew the belt and I think you will get excessive belt wear. The two pulleys should stay in alignment throughout the operational range, and flipping the driven to make it work with the backplate will ruin alignment as the belts shifts up. Probably will work for a light duty application, but if you want to ride it hard with a higher torque engine you may be buying belts.
Okay, so the only reason a backplate exists, is so you can just bolt the thing to your engine. If you are going to fab something up, it definitely would not be a backplate.
Instead, why not consider welding up a rectangular 'box' for the engine to mount onto, then weld a tube to the box. the box could be made out of 1/8" steel (3/16" might be over kill but is an option). The tube would be 1/8" wall tubing that is 1 5/8" OD, so you can put in 1/38"OD x 5/8" ID snap ring bearings and a 5/8" jackshaft to hold the driven unit.
You would drill the motor mount holes in the spot on the box that would give you the appropriate center to center distance.
cheers
Others have made it even simpler (if you cant weld) get a large rectancular tube, drill holes in the tube for the jackshaft, and mount two pillow block bearings on it.
