Jasony976,     Unfortunatly not, the supported configuration on the R350 to have the BOSS and H755 is with the front perc (fperc) version of the controller, the H755f (part number 3KDWX).   Let me know if this helps.    
Wanting to install a Boss controller. Problem is my Perc H755 Raid controller is encroaching into slot. Can I move Raid controller to PCIe slot 1to make room for Boss controller card without any system degradation?
PowerEdge 350. Moving raid controller to PCIe 2