I just made a manifold today. I used a long drill bit approx 3/8" dia. - a rat rail file would work too but I couldn't find mine at that particular moment.
It made The inside look a little better but not good enough for me. I went to the bench grinder with a wire wheel. Letting the wire wheel run the length of the slits really cleaned them up nicely.
I ran a metal rod down the inside to flatten them out, then folded 80 grit sandpaper in half and attacked the slots. Worked well, but my hacksaw cuts may have left thinner burs.