Your tractors rear tires are the size of the fronts on the 990 give or take a few inches. Your tractor was the smallest model in MTD's line up where mine was the largest at the time.This doesn't look as big as your 990. I have 13x5-6 front tires, 18x6.58 rear tires, 8hp briggs and stratton, with a 34" mower deck. I do see that mtd has been sold under many different names.
If you can find the actual model number you can get manuals off of MTD's website.it looks to be similar in size to this MDT 300. Now if only there was a good resource on the MTD's of the '70's.