Since the shocks on the shortbeds mount behind the axle, I would build a strong mount that sits flush with the lower edge of the axle tube and then after I knew what the travel would be, get a shock that fits the bill.
I'm using the stock shocks and am short about 1 inch of uptravel and 1 inch of down so I'm guessing that with a bit more length afforded by a lower bottom mount and a better shock, you can get the full amount of travel. Can't say for sure though as I don't have a ton of experience with a shocks.