I think C3 would be 1963.
Decades...B=50's, C=60's, D=70's, and so on.
The number following the decade letter is the year in the decade, so C3 would be 63.
The A indicates it is from a full size Ford, the E designates it as part of an engine group.
The 6015 is the basic part number group...numbers from 6000-6898 are engine base assembly.
The H is the engineering version...essentially the 8th version of the engine.
Here is where I got my information from:
Decoding Ford casting part numbers
I don't think the 302 was introduced by '63, so my money would be on a 289 or a 260.
I am just curious...how many bolts hold the bellhousing on? I would bet it is just 5, the later bellhousings used 6 bolts. If it is a 5 bolt bellhousing, it would be a good engine with which to build a replica, as some states allow the car to be titled according to the year the engine was built and a 1963 might not require any smog equipment, perhaps only a PVC valve if you are in CA [Edit....just looked, it would be hard to be further from CA than you are!!!]
Others will be along shortly enough with the real data, what I said is just my understanding of Ford's designation and memory of what was available when I was 15 years old in 1963 and just learning to love the Fords!
Cheers!
Doug