There are usually 450-500HP 440 crate engines that have some sort of warrantee for 9k-11K. Here is one example.
Big-Block Mopar Crate Engines
Another on eBay. I know nothing about these guys, just a 8K+ example.
MOPAR 440 ENGINE ASSEMBLY HP HYD CAM IRON HEAD street/strip 500hp ready 2 run | eBay
Then I would guess 4K or 7K to have it installed depending on where your at, if you have all the parts. Putting a 440 back where a 440 was isn’t difficult at all. Quite a few of us on here can do that in our sleep.
The 6.4 swap might be 6K for the engine, does that have a warrantee? Will it make 500HP?You will also need a small block 727 or 904 trans instead of the big block 727, add that to the cost. Then a lot of wiring work, intank FI pump conversion, and hiring the little stuff done for the modern stuff is terribly expensive, probably 4 or 5 times what it would be to put the 440 back it. That is were 10K projects turn into a 100K resto mod.
I will take the 440 over a 6.4 anyway. Our 500 HP 510 with iron heads once averaged almost 18mpg on a 350 mile round trip, with a chit ton of 1/4 mile passes (made it to the finals) averaged into the trip. Well chosen parts 500 HP is a piece of cake. Power is in the heads.