If you want to keep the gun light I would find out the weight difference in synthetic versus wood stock and forend. I had an 1187 12g. Deer shotgun with the laminate thumbhole/forend and wow was that heavy until I switched to the shurshot synthetic.
I know yours is a 870 20g.,but most guys are building them for a lightweight turkey gun so wasn't sure if you could get the synthetic version possibly to keep it on the lighter side.
Also if your minds set on the saddle mount I'd definitely go with one that bolts all the way through. My brother was cheap and didn't and had nothing,but problems.