Sure demodan,
You can do anything you want to do !
When I partially filled my 75 gallon last month, I used a Cap1800 powerhead, with about 15 feet of clear tubing from the Depot, filled most of the 75 from a 50 gallon rubbermaid container of mixed saltwater that I had running for 2 days.
Didn't lift a bucket - and see no reason you couldn't so the same. Delivered 50 gallons of saltwater to a empty tank/with substrate, balance of the old saltwater from the ex-55 added to 75 and sump.
Moving 50+ gallons of saltwater around on a cart seems like additional work.
Do you have an area where you can mix-store-transfer saltwater from this area to your display tank via a pump and tubing ?
You can mix saltwater and keep it in a large container with heater and circulation, and pump from it to your tank for water changes.
I would not suggest keeping more than you would use in a normal water change on hand at any given moment, just mix up what you need, let it run in the container couple few days, hook up your dedicated "transfer" powerhead, run your temporary tubing to your display tank, and let'er go. You can use a siphon tube/hose to do the same for removing water from your display tank, and just run it to drain. I don't lift buckets anymore.
No problem.
To address fshubs comment on cleaning the pump, well .... just clean it after each use.
Works like a champ