The 5th wheel would look a lot more correct about 8 to 10" in front of the axle centerline. A slider is not very period correct for a single axle tractor. An angle mount stationery would look a lot more like it belongs there. They are around. In fact I scrapped several dozen stationery 5th wheels about 6 weeks ago when short iron was bringing $240 a ton. If you aren't going to haul anything, you might just cut the slider off and weld 4" angle irons to it and bolt them to the frame. Here is a picture of a stationery mount.

http://www.fifthwheel.com/onlinecatalog/a36.html