Hi Betsy, Good news! although Mummerts site list that an ECZ block can be 292, I have dismantled about a half dozen ECZ blocks and all turned out 312's so chances are very high that you do have a 312!
The downfall is, parts are scarcer and more expensive for 312's and they are prone to cracking in the main web areas due to the larger mains. Read Mummerts Y block assembly errors page! May want to magnaflux in those areas.
The C heads are a great all around middle of the road head, however the G's, 113's, 471's and B9's have the larger intake valves.
55 Vicky & customline
58 Rack Dump, 55 F350 yard truck, 57 F100
59 & 61 P 400's, 58 F100 custom cab, 69 F100, 79 F150, 82 F600 ramp truck, 90 mustang conv 7 up, 94 Mustang, Should I continue?