The late model brackets from the truck may not be the ones you want for your e body. I don't know
Screw new beackets! Everything is swapable, but, it would be best to swap entire set ups to avoid belt alignment problems.
It doesn't matter if the engine came out of a truck, car or pick up, there all the same inside and out.
I have a '79 318 in a '74 Duster. All fits right in and works fine, no problems. There is one issue many might want to look after and that is the Alt. is a higher powered unit well over the earlier ones.
Many deleate the Amp gauge in favor of a battery Volt gauge. Not a bad idea.
Chyro stated the Magnums use date. The 318 was a year earlier. Before that, it was all standard "LA" useage. The 308 head is a "LA" head.