JW51 your post prompted me to pickup a 2100S core this week. I've had the same experience with my rochestor B. I rebuilt it, no warpage, starts fine, idles fine, but a little stumble going through the RPM's just like yours did. My truck is a 49 but the 235 is a 56' so it will match up nicely.

1949 Chevy 3600
1975 W-25 Hurst Olds
1970 GMC 1/2 ton Fleetside
2010 Chevy Silverado