[Joe's Garage]

You should be able to find a part number and possibly an oversize stamped on the head of each piston. GOOGLE those numbers and it should tell you which pistons they are.

Not only were the bore sizes 389 or 400-specific, the valve angle on the heads differed, requiring different valve reliefs for each displacement.

1st pic is a 389-style with the narrower valve reliefs for 389 heads.

2nd pic is a 400-style with the larger valve reliefs (bigger and further apart).

3rd pic is the dreaded 8-valve-relief style that goes both ways.

4th pic shows the difference in stock 1.5 vs 1.65 rocker arms with the 1.5 on the left and the 1.65 on the right. Notice that the indentation for the pushrod is closer to the fulcrum on the 1.65 rocker arm. That's what changes the ratio.

5th pic is the top of the same two rocker arms, with 1.65 on the right.

Aftermarket roller-tip rocker arms should be marked with a part number or ratio. Again, Google is your friend.

Hope this helps.