Cars, except T-Birds, came with front sump pans, no matter the displacement. It is not a 312 thing.
Rear sump is a truck or T-Bird pan, but they are different and a T-Bird pan may not fit a truck.
No Y-Block oil dipsticks go into the pan. They go into the side of the block for the rear sump applications.
When looking for a truck pan, try to find one which has a baffle. You can also add one if you get a pan without it. There are also 2 different oil pickup positions for trucks. The best is the one with the removable mesh screen, which is not only possible to clean, but is positioned closer to the floor and toward the rear of the pan. I can post pictures later, if needed.
Lawrenceville, GA