You should also measure length from center of spine down to his elbow, so blanket is deep enough to fully cover his sides. I have found the Rambo, Horse Ware blankets to be full cut, for deep bodies, full-figured horses, and cover the entire animal well. I hate blankets or sheets that end before the lower belly is covered!
Proportions on animals can fool you easily when going by eye against the measuring tape!! My 49inch tall pony wore a 68 inch length sheet. She was a Welsh cross, quite full bodied. I had a 14.2h horse thar wore a 78" blanket, girthed at 78". She wore a 5 1/2" bit, wore a cob bridle with an extra wide browband Not excessively muscular like bulldog QH, to need the blanket size. She usually weighed about 900 pounds, smooth muscled. She was a big fooler at the Pony Club when training kids to measure horses. Everyone always guessed her much smaller than she was. Everyone wrote guessing the sizes on several horses, down a list. Then we handed out cloth measuring tapes to get measured sizes on list, to see if they were right. Object was to teach kids not to guess, before buying tack and equipment for their horses. They usually guess wrong. Parents also participated, learned even if horse has a “refined head and body” they can still be a sizable horse!
One TB horse got new bits after measuring mouth. Horse Mom had carried on about horse head being so refined, needed a 4 1/2" bit. Then kid had problems bridling, riding. I suggested bit could be too small, but Mom was insistent on “delicate head” needing a small bit. Armed with correct measuring skills, they went home to measure horse, for a list of sizes when they hit tack sales. Lo and behold, horse measured 5 1/2" IN THE MOUTH!! Horse got new bits the next day!! I was sure glad for the horse. Magicly, then all bridling and riding problems went away!!
I would actually measure length on your pony again, to see if he could be bigger than 70inches. Haffies are pretty full bodied when mature. They use up length with deep chests and muscular hind quarters. We use Quilting cloth tape measures, because they are so long. No having to lift a too-short tape to add length as you work down the body, maybe get incorrect lengths.
She was supposed to be haflinger and quarter horse, although she looked like she had icelandic in her.