273 vs 318

I will be starting a '67 Cuda Convertible soon. I just read the NHRA rules as far as convertibles and roll-bars. If the car runs quicker than a 13.49 it must have a roll-bar. I don't want to put one in an 83k all original car. So...
The car has the original 273 2 barrel/904.7 1/4" combo. I also have a "77 318. What are the pros and cons of putting the 318 heads on the 273(for unleaded gas and a modern aluminum 4 barrel)? I will use a fairly mild Hughes cam and the goodies that go along with it, a low rise dual plane intake, and at least dual exhaust(probably TTI headers). Was also thinking of keeping the 904 to keep things simple,but I also have a 727 that is in great shape. Will this combo have too much snot for the stock rear? BTW, It is a 3.91 sure-grip.
I was originally hoping to have the car in the high 12's, but I am starting to think the more practical cruiser route for this car. I don't think I want to cut up a low mileage original.
Thanks for any ideas and in-put.
SS43