It looks like 6x8 to me. The 5x7 s that you have there dont have the extra tabs to make them fit in 6x8 holes like so many do. If you cant return them theres probably an adapter plate that will do that for you. I will be doing the same thing in the spring to my 98.
They were 6x8's. Ran to Walmart and they had Pioneers, 1/2 off!!!! In and sounding great. Nest is the deck...