Another 240mm would be good, a 120mm should be enough too, but if a 240 i would go for one...
Some say 120mm radiator per block plus another 120mm are enough, but I wouldn't skimp on radiators
The kit includes some barb fittings, for 2 GPU blocks with bridge you'll need 2 more, also 2 more for the additional rad. So you can get whatever 7/16" x 5/8" (size of the includes tubing, ID x OD) fitting, either you stick with barb fittings or go for compression fittings (more expensive, but better lookzz).
You could also 'copy' the kit (put the parts separately in the cart) and replace the barb fittings with compression fittings, same goes for the tubing itself. 7/16" x 5/8" has not that thick walls, 3/8 x 5/8 is a bit thicker (and the inner diameter is therefore smaller) and won't kink that easy.
If you planned the loop and have many bends, 3/8" might be better.
I'm on 2g right now and therefore to lazy to link some examples (loading takes forever)