Bought a '74 Highboy that supposedly has a 429 in it. I really don't care what it is because everything else is satisfactory. But I took a quick look at the heads and the first digits of the casting are C4, which I believe = 1964 production year. Is a '64 head even possible on a 429?

 

No. The 429 was not intro'd until 1968. The 429/460/370 are known as the "385" series, or "Lima" in medium duty trucks.

The prior series was the "FE" ("FT" in mediums) series. That line up included a 428.

It is possible that someone put '64 head on a 428.

Does the intake manifold extend under the valve covers? If so, it is an FE motor. The other easy way to ID the motor series is valve cover bolts:

2 bolts: 239/256/272/292/312 (produced from 1954 to 1964)
5 bolts: 332/352/360/361/390/391/406/410/427/428 (FE engine)
6 bolts: 221/260/289/302/351W
7 bolts: 429/460
8 bolts: 351C/351M/400

The 1964 heads could be high performance jobs. Check this for some of the casting numbers to look for:

FE Cylinder Head I.D. and Application Chart

Actually that only shows one '64 casting number for a high rise head. Unlikely methinks.

More of the same with block numbers and more, again only one '64 number:

http://www.erareplicas.com/427man/engine/partnums.htm

The stock FE for your truck would be a 360. (No 390s in 4x4) Don't be disappointed to find you have a 360 with '64 station wagon heads....

Or you could have a 10.5 CR 428 with bigger valve 427 heads.

Only a tear down will tell the tale.

 

I forgot to state that the valve cover is five bolts. And decoding the VIN revealed that it was a 360 c.i. production truck.

The manifold does extend under the valve cover, something I knew could be helpful but forgot to check for it.

The head casting code is C4AE 6090G.

So that is about the extent of what I can tell from the outside. I looked on both sides of the engine block and saw nothing helpful, but if there's something behind the starter I didn't see it b/c I didn't pull it off to look.
I'm trusting something on the crank will help me figure it out as I'm going to pull the pan to check out the crank and timing chain, assuming I can get to the chain from there.

 

2nd link above has crank numbers, see them here as well. Some are 3 digit identifiers, and it shows where to find those in this link:

Ford FE Crankshaft Casting Numbers - FORDification.com

 

Thanks a lot. I came across a link a few days ago that gave numbers found on the crank but lost it.
Now to see if I can wrangle the pan off. Wasn't aware it was such a chore. May have to wait until I replace the rear main which, judging by the oil being blown back to cover the rear shocks, must be nearly nonexistent.