just checked my yakima bike mounts. Lock Jaw (older style, but no drastic changes to newer ones) stand up mount trays are 52", and fork mount long tray is 43". the way the stand up mount works, the bar spread must be around 8" or so less than the tray length. bottom line, if you plan to carry bikes check your mounts before drilling any holes resulting in large bar spread.
as for the bed platform, i had the exact same thought, but then decided against it. the primary reason is that most our boats will fit completely in and the topper will lock, so it is more convienent to throw the boats in where ever we are going. on the flip side, it is a pain in the ass to put stuff on the rack given how high it is (i have a small step ladder i keep in the truck), so after work paddling and day trips the stuff just goes in the bed, and it is all locked up when we are finished. and with the two of us and a dog, the bed platform would be a little tight for sleeping . i think your toyota bed is a little smaller than my dakota, so if the boats don't fit easily the rack may be your normal choice anyway, but i would say give it a while before building the bed platform. just my experience. later.