If I was going to put an automatic in that truck, I would use a beefed up C6. Basically, the FMX is a cast iron case version of the C4, but more slushy. It is only slightly stronger than a C4. A properly built C4 would be better than a stock FMX, but it is still not the best choice for a truck.
A properly built C6 would be a better choice for a truck. Just remember, unlike the C4, the C6 has the bellhousing integrated, which requires purchasing the correct transmission. The one for the 351W will work for the 302.
I am assuming the truck will be used as a truck, and not a race car.