The 63-66 trucks only have about 25" for the radiator, you can go wider but you need to sacrifice the battery box. I used this one and it cools just fine.
http://cgi.ebay.com/ebaymotors/Chevy...Q5fAccessories
If you need any help, let me know, I put the 4.8 in my 65
Also, there is nothing wrong with your stock one if it is in good working order.