Please understand I have not done this but it looks to me like you need something similar to the plate used for shoulder belts in the AD trucks. Here is a link to show the plate.
http://www.julianos.com/Articles.asp?ID=277
There are several methods to get the plate located. Some have used a stick, a welding rod etc. Just google installing seat belts in AD trucks and you should find several examples. Clay
I realize the examples show the plate going in the back of the cab, but maybe you could figure out something to go in the front.