I'd stick with the 15s if the only other option was 20s but a 16 or 17 inch rim isn't bad.I've got 16s on my 71 K10 with 35" tires..As for clearance if you would go to 35s....With a 3" lift you might run into issues with the front tires hitting the fender cups....I'd run 35s on the current 15" rims though if the 35s would fit..
Here's my 16" rims and 35s:
