not trying to be a "dick" but as has been said there is much more then just Last to consider. The Liner is much thicker in the FT and there is not insole so when you put in a footbed it changes alot. Also the cuff is very differnt. Also if the boot is second hand has the first owner had any work done which may change teh boot from its standard fit.
All I was hoping to show you was you can not know a boot will fit by just knowing the last. If nothing else go to a bootfitter and see if he thinks your foot will fit well in the booter. If he thinks it will work, go ahead and trade, then take them back to him and get them fitted.
However if you have already had the SPK's fitted and there working for you, there is not point in the extra expensive just to have a FT.