Just building a trust like a floor beam. 

It would be heavy.  Maybe use 2- 14 ft 2x6 or 2x8 spliced together to make 28ft. You will need to make 4 of these.  Space them apart with 1x4 or 1 x6 boards. Cross brace, ect. until you have a beam that will not sag.   You will be double striking the concrete as you go around caused by the spacing between the 2 bottom 2x6 boards that are 28 ft long.

Put an eclectic motor on top in the middle with a pulley on it that has a nut or some object on the pulley to make it off center by weight.  When the motor is plugged in it will vibrate and shake the whole beam as you go in a circle striking the concrete. 

Guys that poured my shop slab used a relatively short screed and a laser level to check the height.  Worked out pretty good, I'm sure it's all within a quarter inch.

78' X 78' slab, and all within 1/8" + or -,,,,, I'm happy with.  It's not like there are quarter inch hills and valleys all over the slab, just one area that is a little low.

contractors use a truss screed. 

You will be more accurate setting pins and using a 14' than trying to man handle a large screed.  You need somebody there with a little experience on that large of a pour.