Preparing for rear end change

[NC67Chev]

I want to start preparing to put a wider rear end, so I figured I would explore options of making it a little more highway friendly. My truck has a 250 with a powerglide, 3.73 and 215/75/15 tires. I know there are better transmission options, but I'm not planning on changing it.

From what I can tell, it's turning approx 2940 rpm's at 65 mph.

Here's some other combinations (all based on 65 mph) I found that I might consider:

235/75/15 with 3.73 - 2822 rpm's
215/75/15 with 3.42 - 2697 rpm's
235/75/15 with 3.42 - 2587 rpm's
215/75/15 with 3.08 - 2429 rpm's

So what I'm looking for is some expert opinions on this subject. I realize my calculations might be off, so please feel free to correct me.
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