5" lift blocks? yikes, your gonna have some issues breaking pinions with the axle wrap that will produce at some point.
As for shocks, there is lots of vendors, may I suggest looking at low range off road, trdparts4u, marlin crawler, trail gear or poly performance, some of these show specific shocks per vehicle, but basically you should be looking for a 5" lift shock, and actually rear lift springs or parts for the 63" chevy spring swap while your shopping, as per brands, bilstein have a good reputation for the price.
If your looking for what length to get, best thing to do is see what your max/min lengths are by driving one side of the vehicle up a ramp, second best would be to find the length of the stock shocks and then add 5" (minus some for sag if they are old springs).