Referring to electrical control mechanisms. The reason I ask is that I've found some useful information here.

_________________
72 dart acquired 12/08 /6,holley2b, mopar perf intake, mopar perf header, cam, built motor,904 finally re-installed, 8.75....still needs sorting out.
99 dakota 318 5 speed reg cab picked up 9/09; Trans finally fixed! woohoo!

500= 904 front 1/2 518= 727 front 1/2 (a lil more involved than that but thats the gist)
What you DON'T want is the 96+ (thats when they changed not only the name of the trans to the "RH/RE" designation, but gave the computer control over the trannys governor, instead of throttle pressure. They still use a throttle pressure cable but they have added solenoids within teh valve body taht the <95 ones don't.
Swapping between a 500 and a 518 isnt the issue; its crossing taht year threshold. I had a 518 out of a 92 1-ton van with a 360 (last yr before the 360 went "magnum") it did NOT have a L/U converter either. Mopar Perf sold several of these non L/U versions some years back under the "P-parts" program.