If it was a Gen V motor I was going to tip you off on Gen V closed chamber large oval port heads used on Gen V 366 and 427T truck motors that had the ~97 cc closed chamber and large oval ports used on early non HP Mark IV BBC's.They do have the smaller 1.94/1.66 valves and need to be upgraded to the 2.06/1.72, but will increase the the CR on a flattop piston 454 up to the 9-9.5 range and have the larger standard oval ports instead of the peanut ports.They have the Gen V coolant passage pattern and bolt straight on with standard Gen V head gaskets,they have a 1140 casting number instead of the 1156 number of your heads.I have a Gen V 454 with the 1156 heads that I swapped in to my truck with corvette exhaust manifolds and a 66 360 horse 396 holley intake that while it ran fine stock was pretty tame power wise, I swapped on these heads and put in a ZZ502 cam and it turned the motor into a beast compared to what it was before.Since your engine is a Mark IV you could achieve the same thing by using the 69 and earlier non HP BBC heads, but since you have already converted for use with the Gen V heads you should be able to use the 1140 heads.