That is what is in my 65 318 rebuilt. Paint it red, use 273 valve covers and exhaust manifolds, most cannot tell the difference. It runs better and has much more potential, mine just ran mid 15's a few weeks ago at Eastern Mopar Classic at MIR. Most people there thought it was 273 and I let them think what they wanted!
When you say "most cant tell the diff between a 273 and a 318", what exactly are the tattle tale traits of the 2? Is it the number of motor lugs that attach to the mounts? Thats all I can see, if they are even different at all (273 had 4?)