A518 vs 727

the 518 being the same as the 727 isn't as true as you may think because the later parts used weren't as strong as the earlier 727 even though the gear ratio is the same for the first 3 gears. One of the reasons persons here use a 200 or 700 4R transmission from GM.
727 and 518 use a 2:45 first gear - 200 4R a 2:66 first gear and a 700 R4 a 3:07 first gear.
Using a 2:45 first gear in a 518 and lets say a 4:56 rear, may be able to cruise at 65 mph at around 2000 rpm where as using a 3:07 first gear in the 700 R4 with a 3:23 will come out of the chute like it had 4:56's and the 4th gear will cruise under 2000 rpm at 65 or 70 mph.
On another note: I run a 3:42 rear in my truck with a T56 trans ( 2:66 first gear like the 200 4R ) where-as 2000 rpm @ 70 mph in 5th gear.
At 2000 rpm in sixth its doing 90 mph and at 70 mph, its at 1150 rpm.

Gear ratios of both transmission and rear end can work with each other or against each other. Choose your rear gears wisely along with your tire size...