Am I too tall to bother modifying my old trek? I bought an old Trek 500 series road bike a couple months ago and have put love and parts into it but it's just too damn small for me (I'm 6'4"). All the post adjustments are at their safest limits. Are there mods or extensions worth buying to make it more comfortable/efficient or would it be cheaper/easier just to sell it to a short person and find a bigger framed bike? Thanks, J
If you adjust a bike to the max and its still significantly too small, you should sell the bike and get a new bike. 

There are extra long seatposts, and extra long quill stems and what not, but in all likelihood its not really worth it if the bike doesn't fit with the normal seatpost + stem; the other parts of geometry like top tube length are probably too small anyway.