Ideally you should use an adjustable checking pushrod to determine the correct pushrod length. If you haven't cut the block deck too much you can use '70 TA pushrods. They used adjustable rockers with hydraulic lifters in these engines. As Moparforlife said solid lifter pushrods are way too long. The cup on the 273 solid lifters is set much deeper in the lifter body than it is on hydraulics.