It has more to do with body length to shaft length, in most cases a shock that is 1/2 longer wouldn't cause any problems to just bolt it in and adjust the rear skid to match. an iqr shock is pretty proportionate and could be shortened.
you can sure get various length shafts and thats the best way to do it, We used to use PVC inside the shock to shorten them for drag sleds, I'm sure ya dont want go that route. anyway I guess I'd look at shorter shaft's, Call Walker's I'd bet a paycheck they have some sitting around waiting to sell them to ya.