Exhaust Manifolds

Dan the man said
The reason 318's are discussed so much is simply because they were the most popular engine option until 1974 when the 360's became available in the A bodies. I know that there was the 340 but it was a performance package. For the average person the good old 318 is a very good starting point. If your already has a 318 sometimes it's not practical to swap out for a 360 as you have the cost of buying it, also the LA 318's and 360's are balanced differently

Probably part of it, but also 340/360 builds are fairly straightforward and established what to do, 273/318 there's little consensus about best way forward especially since a large group of people see no way forward other than barely touch it or a 340/360+ swap.