You're shooting a 60 pound Howatt Hunter at 29". Add 3 pounds for the extra inch of draw weight, gives you 63 pounds

I assume your arrows are at least 29.5" long, so add 2 1/2 lbs spine for the extra half inch of arrow length (5 pounds per inch), you need an arrow spined for 66 pounds.

Add 5 pounds of spine for each 25 grains of tip weight over 125 grains. 50 grains over = 10 pounds of spine. You're up to 76 pounds.

I'd start off trying 2216's or 2219's to tune with those Ribbies.