There's a couple ways you can go to give it the big block treatment.

Yes, you could hunt for a correctly date coded 428 block, but as Dave pointed out, I don't think it will add as much value to a clone as compared to a "real" Shelby 500. If you know where to get any 428 for a reasonable price, go with that. If you can't find a 428 you can always build your own by finding a 360/390 block that can safely handle the 0.080" overbore and dropping in a 428 rotating assembly. If you can't find any suitable blocks to do the 0.080" overbore, the 428 rotating assembly will still give you a nice torquey 410.

If it's just the look of having a big block under the hood you're after, you can always just go with a 390 that looks just like a 428.