Redundancy and Overkill time, (best reasonably priced thermostatic cooling fan)

You need to better explain what's going on. You have two pusher fans, but you want your buddy to build a shroud? Where would the shroud go?? On front for the pusher fans, or on the back for an additional mechanical fan?

If the cooling system is functioning properly, all you should need is a properly sized puller fan. If you need pushers, something is wrong - especially at <400 hp.