Well, I was going to give you some tips on the frame the horse is in, but since he is a 23 yr old QH, I think he looks great!!
So, for you, yes to the above on the lengthening the stirrups, or even take them off. Do not turn your feet out, this is a hard habit to break, but your toes need to point forward & don't worry about heels down. Sit up straighter too, you are in a forward position much of the time. I see air between you & the saddle, so sit deep! I tell children to pretend they are glued to the saddle which works for them:)
Also, please check the girth for signs of rubbing. Old horses are like old people & have thin skin that tears easily. I know it won't look as nice, but put a nice sheepskin pad on to keep his skin intact.