6'x8' sound more like a M105 to me. Ask him what size rims/tires are on it and how many lug nuts.
16", 16.5" 5 or 8 lug rims is a 101.
20" 6 lug rims is a M105.
Of course, it could be one of about 30 variants to the two main families listed above also.
6x8 is about right for an M101. Below is the data from the -10. I don't think the M105 is much larger than the M101. It just has a larger weight capacity and the side boards are higher.
Cargo Body (New Style) (M101A2 and M101A3):
Length.........................................................................................................................100 in. (254 cm)
Width ..........................................................................................................................73.5 in. (186.69 cm)
Height .........................................................................................................................19.84 in. (50.39 cm)