Transit seats are all over the place and all over the price spectrum. Where are you located?? (considering adding to your profile). I know of some seats available in the SF Bay Area.
re: Fitting these to an Econoline - The Transit mounting channels are freaking stout and long, and even longer for te larger benches.. The backing you will need to add for the Econoline is up to you. Of course, the channels on top mean nothing without very solid backing plates. Make sure you get the channels with the seats AND make sure the channels MATCH UP with the seat leg hooks. Not all the seats have quick release legs. Some have bolts at the back vs the quick release handles.
The Transit seats have VERY different leg spacing from the E Series benches. The single seats are ~ 8.25" center line, the doubles are ~16.5", and the triples have three legs. Not all the seats have legs that go to the floor. Some have one leg and side mounts that go into the wall or atop the wheel well. Some of the double seats have left or right offsets for the legs. There are different seat cushion widths (narrow and normal??) for singles and doubles. Seat belt configurations seem to be location dependent. Some come over the right shoulder, some come over the left, some have one right, one left.
This all sounds crazy as I read it, but it is the reality I came across looking for seats to add to a Cargo Transit. Oh, and the seat track bolt to the floor patterns are not standard from year to year in the Transit passenger vans.
You will need to drill holes in the E Series floor or come up with some other mounting method.
Transit seats weigh a lot.
All that being said - The legs can be unbolted from the seats and mounts can be fabricated to mount the seats to a custom frame. Again, the crash worthiness will only be as good as the floor backing plates and the frame.
I suggest you have a look at a number of seats before purchasing to get an understanding of the numerous differences.