I wouldn't use the small rods if I was building a fresh 400 hp engine.I've had the small 318 rods actually look like spaghetti after being in my 4 speed dart for a couple years....and it wasn't near 400 hp I'm sure.also the 72 318 was the first year for the bigger rod in a 318 and for that year...
it's been like that since the 80s.i wonder if it's meant to be for the 8.75 7 anna quarter.i pulled a 65 rear apart a while back and it had a tiny carrier,ring n pinoin and spider gears.the rear came out of a slant 6 coronet.