I would like HELP please !!! News wheels for my GMC

[Marder-Did]

I really need your help!
sorry for my english...

I want to change my wheels but I have a lot of difficulties to know if the wheels that I want can be set up on my truck and especially: without modification.
My truck is a GMC High sierra of 1979, it's stock.

In France, the dealers are more accustomed to Mustang (there is some everywhere) and Corvette, the trucks (70 - 80) are rare and often of years 50 - 60. The dealers cannot certify me that the wheels are going to go on my GMC and the sum to spend is : 1890 euros (for 4)
It is expensive if I am mistaken! I am therefore in a dead end.

I looked at all pages of this forum about the wheels or the photos of the trucks. Many owners installed very beautiful wheels of different constructors but I could not know if it was necessary to make some modifications.

Therefore, I want to put wheels 20" 8.5. On the internet site of American Racing, these 2 models could be set up on my truck, to be sure I asked the question but I didn't have an answer. One day perhaps ? But I hope some help of forum members.
I made a summary of the informations on this forum, if I understood well: Wheels in 20" 8 or 8.5 in the front = no modification (but Offset and backspace?) wheels of 20" 10 to the rear = no modification (Offset and backspace??)

2 models please me :
with an Offset of 0 mm but the Backspace?
http://www.americanracing.com/wheels...MC&model=C1500

with Offset of 15 mm but the Backspace?
http://www.americanracing.com/wheels...MC&model=C1500

MY QUESTION: these wheels of 20" are 8.5 you set up them on my truck WITHOUT modification? I ignore the famous and important backspace...

Do the owners can them to tell me merely (my English is very basic) if that is ok or no?

I am very thankful for your possible help.
Thank you to read me.
Cordially
Didier