i just drove my Mercedes 508 van all the way from Belgium to Namibia, going down the west side of Africa. Before leaving i also wanted to fit 7.50 R 16 tires to my van but unfortunately it didn't work out. I found out that the 508 back axles come in two variations, the difference being the width of the axle. My van was a military ambulance of the german army and seems to have the more narrow axle (only a couple of centimeters less wide than the wide one). This makes it impossible for me to fit 7.50 R 16 tyres as the inner tyre of the double tyres almost touches the metal body of the car. I sticked to the 6.50 R 16 tyres, and on some roads the suspension had to work so hard that even these tyres sometimes touched the bodywork inside the wheel arches.
If you have the wide back axle it will be possible to fit 7.50 tyres, but i guess you need the wheel rims of the 608 van. Back in Europe i am member of a Dutch club of ass well 508 as 608 Mercedes cars and many 508 drivers use the bigger wheels. Of course most of them don't drive much off road so they don't have problems with space in the wheel arches.
In the front of the car there shouldn't be any problem. Again i didn't do it but only because that would mean that i would have two different tyre sizes on the car.
If i would go trans Africa again i would still choose for the same van (although it weighs 5 ton and is a two wheel drive) but i would fit the bigger wheels in the front, as the front axle is really the lowest point and every millimeter of ground clearance is important (especially on the truck roads in the Congo's).
I hope my answer is of any help. Greetings,