I would start with a 50 pound spring so that he can get a good pull on it, and then as he gets stronger you will want to go up from there. I have several springs that I use depending on who is on and if the pole is high or low.
That onoe sounds good enough. Since that about how much he weighs, he should get a good resistance with out it being to toght.
Hemi's is 74.7 lbs and has a 150lb spring on his, but he is an extreme hanger ....lol
Dont start you dog hanging off the ground. He should have all four feet on the ground at first. he can get hurt if you start him out high and he doesnt know how to make a proper bite, broken legs and tails are known to occur from falling down from a high pole.
thats what you want to start with a lighter spring, 61 should be good, based onthe sizes listed thats what I would have gone with too. just remember start low and then slowly raise it up.