Do they make a cap so you can have a 42" tall sleeve but the post continues up to 8' ?

You will just need to purchase a normal post cap! We have many options under the Deckorators brand. You could also purchase 2 of these post sleeves so the post sleeve covers the entire post. If you choose that option, you can purchase a post sleeve cap, and those are item #128242, 128240, 128241.