in my experience, a gt rad is not inferior... at least performance-wise.
i've been running one for over three years daily-driven in the hottest area of the country, and odometer diags show that when the fan kicks on the coolant temp drops faster than the stock cobra radiator in my other car...
well the best is the helm factory service manual.
it would cover every trim for the 98 mustang.
as an amateur do-it-yourselfer, i have found these publications to be absolutely invaluable over the years
check out this 76k mile mystichrome coupe from a california dealership on craigslist. they are asking $48k! that blows me away
edit: just noticed last week they had it listed for $50k...