replacing a through the wall air conditioner size problem??

[Thomas Cayle]

Replacing a wall AC, I'd like to buy a certain unit that is not a perfect fit to the sleeve.  Sleeve is 27wx17h x 17 depth. There is 2 1/2 inches vertical difference with ac and 3" width difference ( 1 1/2" on each side).  The sales said he thought with that space it might overheat, even 

though it would be flush with the back grill to the outside.  He said he could maybe have his guys put the ac on a board to take up a little vertical space difference, but he would not take it back if there was a problem.  Is he being overly cautious with his advice, ie. can you leave a little space around a replacement AC as it sits in the slightly larger sleeve without it overheating?? Thanks, I have no experience with this, the units that might fit it exactly seem to be much more expensive.