I did one of mine with two spring bases and M1009 buckets. The bases were $20 from Coleman's.
The CUCV bucket's mounting holes are spaced wider and further from front to rear than the spring base. The M1009 seat's rear holes, when lined up with the rear holes of the spring base, are a good distance to clear the cab rear wall. With the seat all the way back the seating position is more upright than it is in the M1009 but is still very comfortable.
To mount the CUCV bucket to the spring base I used a heavy piece or perforated steel strap for the rear holes and used a hunk of 2-1/2"x1/4" bar stock for the front.
I mounted the passenger side base about an inch ahead of the driver's seat to get clearance for an extreme duty marine inverter behind the seat. Under the floor, the front holes were through the flange of the floor support and needed no reinforcement and I used Grade 8 fender washers under the floor for the two rear holes.
Yanking the seats out of the parts truck, modifiying everything and installing them took about 3-1/2 hours.
Most any seat you put on the GI spring base will be wider than the GI seat so be sure to test fit the side-to-side clearances from the door, the parking brake and the seat belt mechanisms. You may need to mount the new seat to the base an inch or so off to one side.
I'm 6' 4" and my son is 6' 3" and there is ample room.
Lance