‘A’— 4” WIDE 1 1/2” DEEP beginning 3 1/4” from top of  stock rib inner side, and angled down to match 3 1/2” from top of  stock rib outer side.

‘B’— 2” WIDE 1 1/2” DEEP beginning 3” from top of  stock rib inside side and angled down to match 4”from top of  stock rib outer side.

‘C’— 24” IS OVERALL WIDTH OF MODIFICATIONS.  This drawing based on 55-6 but can be used to do 57’s. On 57’s the drivers side rib is 1/2” higher than 55-6 so you need to add 1/2” to ‘B’ rib measurements. Measure 24” from the outer side of this rib to other side of engine and duplicate the mirror image of the drivers side pocket.

Note this is meant to be a guide only. 
The only way to be sure how your engine will fit is to trial fit it.