I ended up putting a piece of
this filter material under my 1262 to silence it. I chose the green for it's cushion feel over the others available. It's a bit more than an inch thick and I use it under 4 different pumps with great results
If it were me, I'd lower the full siphon pipe. This will give you more flow due to more head pressure
My main drain is only about 4" tall and my emerg is a couple inches from the top
48x18x18 is closer to 67g