What to do??
I am going to have to agree with 71produster on this one ... build the 360 and store the 340. If you have storage issues and can't store it then I would build the 340 and take a chance running it but I would never part with a #s matching motor by choice (especially if it were a hi-po motor from a Modtop package). Whether we believe in it or not, the value of a #'s matching car is still significantly higher than its non-#'s matching counterpart.
BTW: Pics would be greatly appreciated.