I've said it many times before...... The best bang for your porting buck on a BB are the 346/902/452 heads, especially if it's your first time.
The MP instructions are to not mess with the short turn, and on a 906/915 intake port you're going to be lucky to get near 250cfm if you adhere to those instructions.
I think the reason they tell you that is because it's really easy to get it wrong.
The flatter floor on the newer style intake port will benefit from a mild blending from the floor into the bowl. On a 906 head, that gets you almost nothing.
It's much easier/quicker to get the newer heads to flow 255-260 than the 906/915(or stage v for that matter).
Was thinking the same thing cos after all, that was a LONG time ago and lot`s has changed in every aspect of hot rodding. FWIW my Vicroy ported 906`s went 278-221 iirc and pushed my junk to 10.40`s and was my daily driver for years..............
