Single or dual diaphragm brake booster on 74 k5 4WD

[Rayplasch]

Replacing original brake booster in 74 k5 4wd.do I get single diaphragm or dual diaphragm?was original single and is dual a upgrade .need to order and not sure what to get

 

[Wes Harden]

if you have front disc and rear drum, single is fine. front and rear discs get a dual, or if future rear disc conversion get a dual