It is a pretty involved question,and it "depends on who is asking!". ;-)
The FIA Group 3 specs I received show that a 318 was legal in 1966 for the Barracuda, I guess because the engine was in development at the time and Chrysler holomogated it that way. Group 2 specs used the normal production 273, but with the D-Dart performance "go fast" parts in it. To keep up with the vintage racing 348 CI "289" and "302" Mustangs and 355 CI "283" and "305" Chevys I ran a built up 340s (disguised as a 273s), in the car in the early days (mid 90's until about 2005), and broke things quite frequently (dropped valves, broken pistons, etc.) I finally blew a hole in the block big enough to see the ground from the top while in a race at Thunderhill Raceway in CA in about 2005. The piston and rod fell out of the side of the block at the #7 cylinder at 6,900 RPM at the end of the straightaway, and literally cut the block in half. Also cut the starter off the car, which was dragging on the ground. End of race, end of engine, end of racing season!
(As an aside, I towed the car to the So Cal Spring Fling Mopar car show with the "well ventilated" engine still installed, and there was no question for the judges as to the fact that the car was actually raced. Won a First Place trophy!)
I then went with the MP 4 bolt main small block (basically a modern T/A block), with thicker webs and cylinder walls, and a 318 rough bore. My new engine guy knew all the "secret sauce" tricks, had a 340 forged crank cut down to use strong Chevy big block H beam rods, and had custom forged pistons made up.
So how many cubes now? For the race tech inspectors it is 325 CI, (.040 over a 318, the max legal) but in reality it is just a bit bigger. Compression is about 11.5 to 1, and the cam is a custom ground Elgin mechanical tappet model designed specifically for the engine. Heads are heavily ported J parts, with Chevy sized valves (narrower stems for more airflow), 2.02 and 1.65. Shifting redline is 7,000 RPM, and HP is probably in the 400+ range. Guys are getting more HP out of their Ford and Chevy powerplants, but this engine has been in the car for about 8 years now without any issues, and probably has enough track miles on it to have run the 24 hours of Le Mans.
The bad news is that my engine builder died very suddenly in 2007, and some of the recipe for the motor died with him......
In road racing I have found that lap times are more related to driver skill and aggression level, (willingness to take risks), the handling of the car, and the brakes, all of which have been upgraded on my Cuda over the last 20 years. These will make up somewhat for a lack of brute power, except on tracks with long straightaways. When I started off vintage racing I was a back marker, and now the car is mid-pack with other similar small block cars. I continually get beat by L-88 Corvettes and Cobras, but they are not in the same class, so who cares.... For me, vintage road racing is about having fun with the car, and "bringing it home!"
Rather verbose, but I hope it answered your question!
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