I just went through this with a forum member and 428CJ heads. He wanted me to rework his heads and port them some. When I receive a set of heads, I always flow them to see where they are as received. Then whatever modifications I make, whether valve job, oversize valves, porting bowl, chambers, or competition port and polish is completed, I re-flow the heads to see what, if any improvement was achieved with the modification. The heads above were flowed out of the box and only flowed in the 235cfm range @ .500" and did not pick up through .750". When I pulled the valves out and measured them they were 2.190" valves in a 2.090 seat, and 1.750" in a 1.665seat. Just by sticking a stock CJ valve in and reflowing, it picked up nearly 15cfm, no work involved. I flow on a 4.200 fixture, or a 4.350 fixture for FE heads. Unless the walls of the chamber are laid back to the bore size, the flow will diminish and not change above a certain value. A case in point is a set of heads I did for this same customer a few years ago, and they flowed 313cfm without a valve job and using a 2.150 intake valve. He wanted a local machine shop to do the valve job and install a 2.190 intake valve. The car did not run any faster, and when the heads were sent to Blair P, they only flowed 260cfm with the oversize valve. I have had other heads flowed before Blair, and after Blair's bench was used, and our flow benches are very close in cfm figures, so I know the larger valve job killed over 50cfm. Just my own experiences with FEs, I have seen very similiar results on SBFs and brand X. Putting a MR head on the 4.130 bore is doable with the intake, but the exhaust needs to be chamfered similiar to the 427 block if you are going to use anything larger than the 1.665 valve. You might squeak by with a 1.710, but the block needs attention before I would feel comfortable with it. The flow difference is negligible between the 1.710 and that bore, and a good 1.665 valve. Joe-JDC.