In the rear you could upgrade to a 14 bolt full floater which is probably the strongest rear you can easily swap into your truck. Usually they go for about 150 bucks and one from a 3/4 ton truck is a direct fit (just need to get a conversion U-joint). The problem with this axle is that it is 8 lug as opposed to 6. You didnt specify if this would be a problem or not but if it is and you want to stay with 6 lugs then you need to look for a 14 bolt semi-floating rear axle. I think that there are a few out there that will bolt in but most will need the spring perches relocated (which is not that difficult). A 12 bolt rear would only provide a marginal amount of strength more than your current 10 bolt rear.
The choices for the front are more limited. You can either stick with your current 10 front or go with a Dana 60. Swapping to a Dana 44 front would only provide a bit more strength over the 10 bolt (if any at all) and is not worth the effort). Going with a 60 front would mean needing 8 lugs and makes the rear axle choice easy, 14 bolt ff.
The cheapest option may be to swap in a 14 bff in the rear and put 3/4 ton knuckles on the front axle (gives you 8 lugs and larger brake calipers)
Something to consider when going with larger tires (over stock and especially in 1/2 ton axles) is the the additional rotating mass that is added. Your current brake system was not designed to stop this additional mass. That said, you indicated that you did not intend to go over 35" tires and i would say that sticking with 35's or smaller would probably be fine on 1/2 ton braking systems.
1973 K20 - 350/350/205
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