As I browse through racing junk looking at the conversions, some seem as though the rear axle is almost at the end of the conversion, others seem like they are set farther forward of aft.

What determines where the rear axle or axles are located? What are the variables or factors which determine the location?

Does the manufacturer determine this or is it negotiated with the buyer...?

There are advantages (& disadvantages) to both (forward & rear axle) placement.
Forward: better turning radius,
Rear: less chance of dragging the pavement on a steep incline, better trailer handling (IMO)

Supposedly the longer the wheel base, the smoother the ride. The only possible evidence I have for this is when I was driving private coaches (buses), we had an axle break on the bogey wheels (which were behind the drive wheels). We limped to a truck stop to have the bogeys removed and chain up the axles. The result ended up in a ride so bouncy that the drummer got car sick. I didn't know that the bogeys ever did more than hold weight, but apparently they are key to stabilizing the bus. They old GM buffalo didn't have bogeys and they didn't bounce like that, but the drive tires were further back.

Not quoting facts, just personal experience.
Doc Weaver

...if you unload[move the rears back] the rear you will improve ride....but if you overload the front you got 2X the problems.....again it all about ride and weight....closer to the max on the front axle the more problems encountered......geofkaye

One other thing to consider is how strong the frame is. If you move the axles rearward then the fram flex will increase and you may need to reinforce it. I have seen some conversions that showed severe signs of the box flexing becouse of the distance in between the front and rear axles. The ride is better with the greater distance but that is becouse the frame and the box flex alot, absorbing alot of the bumps in the road.

Rear axles further back is better for those heavy stacker trailer tongue weights. If the rear axles are too far forward, the tongue weight would un-weight the front axle too much. If they're further back, the rear swing when turning is reduced ... but the turning radius is also increased https://www.truckhome.com/infopop/emo...icon_frown.gif