You are most likely correct of excessive blow by on the rings....
You should also realize that a 230 in a 1965 C 10 will never be a super performer on the highway, given the weight of the truck and the original design use of the engine...
You could do a rebuild on the 230 reasonably cheaply, dont be surprised if the rings in those cylinders you mentioned come out either broken or with super wide gaps....I have seen engines bored oversize and standard rings put on the pistons !
Anyways, thats my 2 cents worth....
