My vote is for the bathtub. Actually, I would test one of the blocks. Fill your kitchen sink with hot water and drop in the block. BTW a great hand dying blogger did a test on different products for bleeders and she found blue dawn dish detergent worked just as well as Synthropol for suspending the excess dye in the water. So test your block in LOTS of water. If you find it bleeds then do your quilt in the bathtub with loads of water but one one hint is to lay a sheet down under the quilt. Once you drain the tub you and a helper can pick the very heavy wet quilt up by the sheet and not put undo stress on the hand pieced seams.