i think the gx1 shafts are going to be too short to fit the FF. i know that cen tr4 shafts are too short, if that helps...?

ive read 2 or 3 people have had issues with the one way diff - they seem to break very easily, maybe worth a look if u have the cash, but i wouldnt do it if u just bash ur truck and dont race. seems like a lot of money for a one way bearing in a casing too!

rubber O rings or a tiny slice of fuel line in the diff cups does wonders for the life of the shafts by helping prevent them binding in the cups - worth a try if u didnt already.

as for finding the best prices - just type "cen ffs004" into google and see what u can find. personally i use ebay a lot for my FF bits, cos cen spares are overpriced, frankly.