ac manifold not sealing to comp. on 1994 Chevrolet G Series Van (G20)

relacing ac compressor and having problem with manifold not sealing to comp. should i have flat washers or oring style

In many cases re-manufactured AC compressors no longer exactly fit the vehicle's hose manifold. You should have received a seal kit with your replacement compressor. You will need to choose the appropriate seals from the kit. I doubt very much they would be o-rings, most likely they will be the flat washer type. Choose the flat washers that will create a level mounting surface for the manifold when they are placed in the back of the compressor. The locations for the washer seals are commonly of different depth so you will need to choose the correct sealing washers so that the manifold will mount flat on the sealing washers.