21" oval vs 14" round air cleaners

either one will be fine. the real difference is how long between filter changes, and the larger the filter, the longer time between changes.

If you ask me, a single 4 bbl with an oval aircleaner just isn't kosher. It's like trying to fool someone. But thats just me.

The drop base, raised lid type is the most efficient.
It arcs the air flow over the choke horn and down into the venturis.
Other types make the air flow make a 90* turn into the choke horn.

Moroso type air cleaners are the best example, though I think this type is made by many companies.
They are usually 14" round.

I have an old Nascar surplus version.
It is very light aluminum, the drop and raised portions are very pronounced, and it is 14".
Moroso recommends a minimum of a 3" tall element.
K&N type is what I like, though I have seen no unbiased info that they make a lick of difference.