I don't recall the thread size on the frame end of my 72 K20 but i bought new hoses from Offroad Designs for my truck after i installed my 4" lift. This is the part number they sent me, if your still searching maybe a call to them would help id the thread size?
(GU30071) BRAKE LINES - VEHICLE KIT (‘73-‘78.5 -- 4-6” lift) (2 fronts, 1 rear)
http://offroaddesign.com/catalog/braidedbrakelines.htm