Jamie - I'm almost certain i tried running 48p but couldn't get it to fit.
Can't tell you what fits or not but it's not low enough. I run outdoors on dusty/dirty tracks that kill gears hence my preferrence for 48p.
My pitbox is full of 48p pins and spurs that i cna't use with this car. However, take solace in the fact that you only ever need a 26 or 27pin and a 110 spur.
As protc3 said - when calculating ratio's the pitch does not matter. HOWEVER!!!!!!! the GTB is afterall, a low turn modified which requires fairly precise gearing. The jump between ratio's on 48p are just too big. You're either too low, or too high. Another reason why my toolbox is rattling with 48p stuff..
The HPI spring kit comes as a full set. Nice springs.
Hope that helps mate..