yes, a crank girdle will go a long way to helping the bottom end.
XRQTOR -
a pair of rb25 turbo's will see your planned 300fw kw,
but the vg30det turbo, have the advantage of future upgrades and more power potential,
yes you can cut down coolers, it's easier to cut them down over their length (thru the cooling fins) then it is to cut the core verticaly,(thru the air passeges), jeez i hope i explained that well enuf lol.
but best to grab a 600x300x75/90 mm cooler of the shelf they range from 700-1500, depending on cooler core brand and fabricator.
the one ebxr8240 suggested.. hybrid i think, they are a pretty decent brand, and last time i checked they were asking 900 for the 600x300x90mm cooler.
using two mazda series 4 rx7 coolers will mean you have that large cooler, but at only 500 odd bucks, problem is they never will work as well as one proper made custom one.
cheers.joe.