Does anyone know the belt numbers or sizes for this mower. I believe it's a 1991 model from reading other posts on here. It has three blades, six speed with high and low and a 46" deck. It is tan with the big rear tire

Hi,
Try the 754-0349 and 754-0350 for top and bottom respectively. All MTD belts begin with "754"
Peace,
Clarke

What size is the deck belt for the mtd gt 18.5 46 inch is it thje 1/2 77 or what

Hi,
Well, if you want to do the rest of the work, like looking up which version of the 46" deck your machine has. I know of four that MTD uses. Here is where the cross reference is:
MTD Belts Listing (http://outdoorpowerinfo.com/belts/mtd_belts.asp)
Neither of the ones above are the length you gave, but you have to start with a model number to really know.
Peace,
Clarke