I've built a gob of pyramid bows and I tillered and reduced weight on most of them by thinning the belly (within reason). Once in awhile I would would removed wood from the side to reduce weight or adjust tiller, especially if the bow was particularly wide or the limbs were getting too thin to continue removed wood from the belly. Some swear by the rule that you reduce weight by taking wood off the sides and adjust tiller by taking wood off the belly. I agree, but it's not the only way. Either way, taking wood off the belly is the quicker (and often more dangerous!) way of getting both done, but you can get carried away in a hurry. Remember the rule that if you double the width of a limb, you double it's strength. If you double the thickness of a limb, you increase it's strength eight fold.
Now, for a basic tillering review: You worry about the tiller first, never drawing past 1) perfect tiller, 2) your intended draw weight, and 3) your intended draw length, in that order. If you're pulling it 60# @20" but don't have perfect tiller, you're causing undue set. In your case, I would take wood from the belly, assuming your limb tapers are spot-on and don't draw it past where the tiller is true. Remember to exercise the limbs between each wood removal.
Post pictures, have fun, and be patient!
