The hyd. roller lifters will fit in an RB block with no problem. They are expensive, the cam costs a little more, you will need better springs, bronze dist. gear, bronze fuel pump pushrod, cam button and shorter pushrods. I just installed the XR292 in my 4.25 stroke RB and it is a good sounding/performing cam, worth the expense to me. The cam is still relatively mild and you may be able to get away with 3.55s, I'll be using 3.91s though.