Long sitting/no usage, rear axle

It's your choice but I have ran the XE274 cam in a 360 and it was no torque monster so I totally agree with grumpus that a 2.94 gear isn't enough gear, unless your running super short tires (24"-25" max). You stated your running Vmax lifters to reduce the lift because your running 1.6 rockers? I hope that isn't because the cam lift with 1.6 rockers is more than the heads want because the Vmax lifters pump up at higher rpm and restores all the duration and lift. As mentioned Vmax lifters bleed off some duration and lift at low speeds but did you know that running 1.6 rockers gives you the same effect as running a cam that has 4 degrees more duration at .050" lift and adds about .025" lift? I have measured and verified that, not just hearsay. The Vmax lifters will help with low speed torque output but by running 1.6 rockers you defeated their purpose IMO. You say the car won't see any track time for a long time so why not run the 268 cam instead? With the 1.6 rockers it'll act nearly the same as a 274 with 1.5 rockers.

Also keep in mind that first gear isn't the only gear you have to take into account. 4th gear OD combined with 2.94 gears will make for a very low rpm hwy gear, which I realize is good if you cruise the interstate at 80 mph but it may be more than the cam will like and it will kick and buck like a Missouri mule in OD unless your running at least 70 mph. Again, the rear tire size has some effect on the final drive ratio.

Whoever says the 68 340 4spd. cam is bigger than the XE268 cam apparently hasn't compared ALL THE CAM SPECS OF BOTH CAMS. The total duration of the 68 4 spd cam was a little more than the XE268 but total duration means very little compared to duration at .050", which is the same as the XE268 and lift which was about .020" less on the 4spd cam. The biggest mistake I see a lot of guys make on street cars is to over cam their engine. Then they fight tooth and nail trying to figure out why it won't make any low end torque. IMO your always much better off going one size smaller on the cam than one size too big

Now back to your original question, the clutches in the diff should be fine as long as their not worn out already, you said it was ran real hard so it's hard to predict what condition it's in. I'd also be worried about the nearly 40 year old seals. I'd bet their dried out hard as a rock and it'll leak oil when in use. Synthetic lubricant's also seem to find every place to leak from they can. Personally I wouldn't install a rearend of such questionable condition because I hate to R&R them. At a minimum I'd do a complete mechanical survey to see what it's condition really is