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What does the diff cover look like, and how many bolts? Round, with 10 bolts, 8.25 rear. Octagonal, with 12 bolts, 9.25 rear axle. The 9.25 axle came stock in full sized cars during the sixties/seventies behind a 440...... So, pretty stout axle.
OK, a picture, as promised. Looks roundish with 10 bolts, so I'm assuming this is the 8.25 rear.
You are correct. 8.25 or 1/4 ton axle. I am assuming spring-over-axle 2WD.
Would be fairly simple to swap in a 9.25 1/2 ton axle. It's structurally similar to a Dana 60 (some tools fit both) and would handle what you plan to build.