I have a 68' Arrow, and wonder if I can remove the back seat to allow some
bulky transportation.
Or is this seat some sort of stiffening of the airframe?

Yep, that bench seat comes out -- but it will expose lots of stuff that
you probably don't want to drop things in...like your elevator and
rudder cable runs. (One reason I REALLY like the "bucket" back seats
in our '74 Piper PA28-235 Pathfinder. They literally pop out --
without tools -- in seconds, and leave a nice, flat, 3/4 inch plywood
floor.)

If I can, can I remove only the seatback? If so, how do I do this?

I don't see why you couldn't, although I suspect that the seatback is
pretty permanently attached to the seat bottom. Any mod you make is
going to have to be approved, which may not be a simple matter.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"