PART NO.  SPL.PAGE  DESCRIPTION                                                           
AHH5065   15.A.17   Bracket -engine mounting (R/H front) -(engine mounting, 1500)
AHH5065   16.A.16   Bracket -engine mounting-front -R/H -(engine mounting, 1600)
AHH5065   TC.A.22   Bracket -front engine mounting (R/H) -(engine mounting, Twin Cam)
AHH5066   15.A.17   Bracket -engine mounting (L/H front) -(engine mounting, 1500)
AHH5066   16.A.16   Bracket -engine mounting-front -L/H -(engine mounting, 1600)
AHH5703   TC.A.22   Bracket -front engine mounting (L/H) -(engine mounting, Twin Cam)

AHH5703 Bracket fits only MGA Twin Cam engine, because the front three holes are in different locations.

Twin Cam Left Engine Mount --- Pushrod Engine Left Engine Mount

Here is an illustration of the difference, using a traced outline overlay. The right flange, facing the engine block, is same for both parts. The left flange, facing the front plate, is different. The smaller outline is the standard bracket. The larger outline is the Twin Cam bracket. The top hole in left flange is same for both brackets. The two bottom boles (closer together) are for the standard bracket only. The center hole to the left is for Twin Cam only. The bottom left hole (overlapping) is for Twin Cam only.

To make a Twin Cam bracket from a pushrod engine bracket, you need to add material to the left flange (creative cut and weld). Top hole is unchanged. Center hole is to be added. Bottom left hole needs to be deleted and added back in new location slightly farther left. The bottom right hole is surplus, can be deleted or left in place.
See additional information at TC-331A.