Without any more information, it is impossible to say, but I would think that if you had bought an existing spar from someone else's boat that would need to be cut down to fit your boat, you have to ask yourself why the spar was being sold - is there a reason they replaced it? Folks usually don't just buy a new spar without good reason! So given that variable, and then , as you said, having to change alot of hardware around, get a new masthead, etc. you are looking at alot of work put into a spar that may be questionable to begin with. And then you will have to deal with old mounting holes, etc. and if you are welding you have to deal with anodizing loss - then you might want to paint it - you might as well get a new one and do it right.