Personally...I would say 3.08. I think the 3.08 is a better fit for your big block anyway. The 3.08's are pretty common and came stock on quite a few models so it should be relatively easy to find one locally.
Option 2 is to change out the T400 for an overdrive tranny like a 200 or 700R4. This would have the benefit of keeping your acceleration ( actually better) , but yet giving you a gear reduction in 4th gear. Have your cake and eat it too. If you like to go quick, the swap from 4.10 to 3.08 will bum you out. If you decide to stick with a rear gear swap, you might consider something like a 3.42 or 3.31.
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