5.7 heads on a 6.4 short block?

If it is in fact a 2003, there are a variety of small differences that you'll need to address. The cam, timing chain, gears, tensioner, etc won't work on the 6.4 shortblock. You'd need to check but the front cover probably won't work. The accessory pulley depth changed so all the 5.7 accessories won't be correct to work with a 6.4 damper. I'd suggest doing a lot of research. A vvt 5.7 would be a much better donor to finish out the 6.4 short block.