That's a perfect amount.
I would just do a tank transfer then over the course of a day or so. Add all the base rock you like in addition to the rock you have. I would for sure use all new sand and not crushed coral, it can trap food and other gunk.
I would then test your water over the next week or two and make sure there are no spikes in ammonia or nitrites.
I'm doing the exact same thing now with my 60g tank. just moved over the live rock, added some base and put the livestock in. so far its been a week with no spikes and i don't expect to have any.
The only thing I wouldnt do is add new live rock from the store to the tank, THAT can have die off and spike your ammonia possibly harming your livestock.