You need to figure out how much water your 55 can hold by measuring how much room you have at the top. I think a 48x13 tank will have 2.5gal of water per inch of water height. So if you have 2 inches of space (I'll call it headroom), you can instantaneously dump 5 gals with no problem because you know the 55 can take it without overflowing (assuming your overflow continues to drain).
Or you can dump 7 gals, but do it at a slower rate so that enough water has gone through your overflow that no water spills over from the 55.
You can actually calculate the amount, but your numbers are only as good as the numbers you use for the return (gph), actual overflow capacity (any elbows or other restrictions can restrict the flow), size of pipe from the surge tank, height of surge tank from water, etc.
I'd just figure how much more water the 55 can hold. Then size the surge pipe to dump all the water in 30 seconds.
Since you have an extra 20 gal capacity in the sump, you are okay sump-wise. I also do no think that you will be dumping 20 gal of water in the 55 - because that means you only have about 35 gallons of water! Dumping 5 gallons in 30 seconds is pretty good for a 55. Try it with your 5 gal bucket (just manually dump it in water). Then you can get an idea of how much water and how fast you want it.
