Question about rims/tires and lowering setup

[ATrain707]

Hey guys first off I'm new here. I bought a 68 c10 long bed for 1000, with a straight six 4speed the body is fairly clean, the floor of the cab has some rust by the the doors which is common I hear. It runs great. My question is I bought a set of rims they are 20x8.5 the lug pattern is the same but I'm gonna have to use 1in adapters to get them to fit cuz the offset isn't right. The tires are 275x45x20 what size tires should I run and how low can I go? I don't want to have problems rubbing. I was thinking 2in in front and 3in in the rear but I would like to do 3in in front and 4in in the rear with air shocks in the rear but not sure what would be best with the size rims I bought, I bought the rims cuz I only got them 300. I'll post pics of the rims and truck later this evening
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