I would not recommend replacing a sit down with a stand up in their application. Stand ups have their special places but I would think this is not one. I had a customer (in Texas) with a Dockstocker that I did some training for. They had taken a tow strap and wrapped it around the back of the unit so as to "help" keep the operator in place. They had a small location and travelled in and out of a (OMG...wait for it...) concrete culvert.
Scary stuff, guys be careful whichever way they go........