will a 518 work here...

I don't know about how to adapt A-518 to a big block but on a mid size vehicle such as a diplomat it makes about 5 or better mpg difference. I own 2 dakotas with one with A-500 and the other with A-904 both 4 wheel drive and the one with A-500 transmission gets an average of 6 mpg difference, not sure if its because it has lower milage then my other one or not Just my opinion on it