I am looking at a GMC book. Fitment is 250CC,EC 280EC 300CC EC280 with 7.00x20 Front tires. These are 1 ton 1 1/4 ton and 1 1/2 ton. It appears that thee are 3 different wheel opening sizes for those years. It appears you have the middle sized one. I can't open the link. A Hollander book is really better in situations like this, but the oldest one I have is the 23rd edition that starts with the AD models.