RE: understanding prop size on my saito powered rambler 45----help
I, personally, would go with the 13X7. But then again I don't have any Saitos.
As far as the crank vent goes, try to keep it short. So no, you don't want to run it the length of the fuse.
What you can do is attach a piece of copper or AL tubing inside the cheek cowl (this plane does have cheek cowls right...?) and extend it down below the fuse two inches or so. Bend whatever radius you need in the tubing to be able to connect the top of it to the vent nipple with a piece of fuel tubing. If you nose over the metal tubing will bend - no harm done. Oh, use a slightly larger diameter CU or AL tubing than is commonly found in fuel tanks for easy breathing (5/32 or 3/16").
I've seen similar things done with other Saitos since they do "weep" a fair amount - weeping is not a bad thing!