Hi, I did the same set up on my 4 seater, keeps the little ones from complaining "he's touching me".

I bought my base from the guy I got my cage from off ebay, he still has the cages (item #250224629762) on ebay, but isn't showing the base anymore. I bet he still has them though, it was fairly cheap like $80. You could always ask if he still has any.

There is one guy on ebay listing a seat base Item number: 150224874484, but it mounts different then mine did.

As far as the suspension I just turned my shocks as stiff as they would go because of the extra weight, seems to work. I made bed locks on both sides, to keep the bed from opening on accident. I just took some 3/4" alum round stock, tapped it and threaded it using the same bed tie down hole $5 project.

 

Got my bracket from Rhinogiant.com. Seats matched up perfectly.

 

Beard has one specific for the stock seats...