I usually look for the 'china wall' above the distributor hole. On a 302, the china wall is no more than a quarter inch taller than the distributor boss, almost flush, on a 351, it is about an inch or more.

302
http://www.fordpower.net/main/images...boss302_04.jpg

351
http://www.adperformance.com/images/FMS-6010-W351_b.jpg

Brian
EFI-Unlimited