The width of the floor of a Series Rover (down between the wheel-wells) is right at 36"... a 35" Jeep box will fit in there w/ a touch of room to spare.
For a 109", from the center of the rear axle to the end of the bed area, should be just over 39". From the center of the rear axle to the front spring hanger for the rear axle, it is about 24". Those together will be close; however, the spring hanger is a bit further forward than the back edge of the seat bottom, and the back of the 2nd row seating will extend into that area.... I'll try to remember to take a tape measure and measure the exact dimensions for you with a seat in place, so you'll know the volume back there.
I love Series Rovers; but don't expect it to make a great family vehicle. If you're family is used to spartan, is going to be in a remote wilderness, never running in highway traffic, it might work out okay.... it makes a great change-of-pace, is out of the ordinary, but don't plan on it to be a practical daily driver.
FWIW......
-L