Safety is the first concern when retrofitting seating. I did this in my old coach and love the leather seats I transplanted from a Chev van.
If you have to use a plate to adapt your new seat to the old base make sure that it is very stout. Seats take a lot of pressure...no I don't mean you're fat...but the g forces of everyday driving let alone panic stops, make for more force than most give credit.
I'm getting ready to replace the seats in the Airstream and am looking to one eBay vendor for the same solution as you. I want integral shoulder harness type seat belts.
I'll let you know how I fare next week.
1984 28' Funeral Coach
Former Rolling Showroom & PuttLab (now party bus)
"I'm not an expert. But I did sleep in an Airstream last night."