Here's more info on the sorbothane cushion, you can contact the original poster for help.
http://www.airsoftcanada.com/showthr...hlight=Tirador
I'd say if you can, get an aluminum piston head. I'd personally be more worried about a polycarb piston head breaking at 500fps than a G&G M14 mechbox failing. My prefered brands are Modify and Prometheus.
As for hopup, what you've ordered is the hopup rubber sleeve and nub. The hopup chamber = hopup unit = came with the gun itself.