Easiest/Cheapest way to a 4bbl 318?

8 3/4 has a removable center section that installs from the front 7 1/4 and 8 1/4 both have a cover on the rear, the 8 1/4 looks normal size (pick up, van, mustang) the 7 1/4 looks like a tiny toy rear

one more inexpensive solution for your 318 would be to use a set of 5.2, 5.9 magnum heads they have smaller chambers to boost your compression, bigger ports and valves to increase flow, and you can use an edelbrock air gap or the cheaper chinese knock off crosswind aluminum 4-barrel intake (I use the crosswind with a set of U-PULL IT junk yard heads)

http://www.summitracing.com/parts/DCC-5007477/