Go ahead and plumb them in (your water chems are good, right? ). The heater needs to be in place to hook up the gas line - the high altitude oriface should be easily accesible anyway. The electricians also need to have the box for the SWCG mounted, unless they're doing it, so that they can run the wires to it. Trust me, these folks are used to having to do a little 'contortionist act' to get the job done (If you want to be nice to them, leave a cooler full of ice, bottled water, Gatorade and a few beers for them)