Ideally, the length of the unit at the pivot point of the tie rods would be the same as the distance between the inner mounts of the lower control arms to eliminate a bump steer issue. The rack should also be mounted so the steering arms at ride height are level with the tie rods, again for bump steer issues.

It may seem dumb but which unit you use also depends on whether the car is front steer or rear steer! A front steer car with a rear steer rack would turn left when the steering wheel is turned right, and vice versa..... It seems basic, but a local guy here actually mounted the rack in the wrong position----would be unique, but certainly present some "challenges" to driving the car!!!!

As for the splines on the column, you'll need to know the spline count and the OD of the shaft, there are a lot of different adapters made to go from the column to a Double D shaped shaft. If there's not an adapter made to fit the splines on your column you might have to do some machine work on the column end and weld (TIG preferred) an adapter to it.

Eliminating bump steer problems is probably the biggest issue on converting to a rack. I use a Bump steer gauge which is a flat plate that mounts to the wheel flange, then dial indicators, one mounted on a fixed stand at either end of the plate to make sure the wheels on the car don't turn in or out as they move through the range of suspension travel. Adjustments to the mounting height of the rack, and/or spacers between the steering arms and the tie rods can correct any issues.

I'm not all that good at putting down in words what I do in the shop....I hope my "guidance" hasn't just added to your difficulties!!!!!!