Are you planning to install a full-size 5th. wheel for semi-trailer type loads (80,000 lb. GVW) or a smaller one for towing something like a horse trailer, etc.? The purpose for carrying the trailer kingpin weight slightly ahead of the rear axle is to allow some transfer of trailer weight to the front axle. Max allowable 18-wheeler axle weights are:

front: 12,000 lb.
drive axles: (dual drive) 34,000 lb.
trailer axles: (dual axle) 34,000 lb.
total gross weight: 80,000 lb.

We would slide the 5th. wheel forward to put the max weight on the steer axle to get legal over scales, then slide the 5th. wheel back for easier steering between scale houses. Sometimes a trooper with a set of portable scales in a rest area would rain on our parade, and we'd get a ticket for overweight on the drive axles.

If you're setting up a gooseneck ball hitch or a small 5th. wheel for a travel trailer, put the pivot point just slightly ahead of the rear axle centerline for the best trailer handling. Going behind the axle CL will take weight off the steer axle, and make the handling a little squirrely at times. It also does funny things to trailer tracking while backing up.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!