As any of us who have worn a racing harness can tell you, a retractor isn't absolutely necessary. Just pick a suitable anchor point for the shoulder belt, and adjust it manually. You'll probably need to reinforce the area where the upper belt attaches to the cab. Summit or JEGS racing should be able to supply any kind of custom belt arrangement you decide to use. Simpson is one of the brands preferred by most round track or drag racers.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!