differential gearing

[WyattEarp18]

I've got a 79 k20 camper special 350/th400/np205. Before I converted to the 205 my max speed was around 60-65 without worrying about blowing the motor. After the swap I'm down to around 50-55. The 205 came out of a 80 k30. Just wondering what my rear gear ratio is? I read they came with 4.10 or 4.56 rears. Reason I ask is im looking at adding 4-6" of lift and 33 or 35s on it. I'd rather not replace the diffs but I'd also like to be able to do 70 on the highway. Any help is greatly appreciated.