If its cracked then just replace it with another Cleveland and dont worry about those serial numbers as according to the last 5 years at the barrat auto auctions posted numbers on types of cars and prices. The concours restorations garnish some good cash but the really nice restomods garnish all the big cash.
I would personally just make it as correct as possible without jeapordizing longevity with sleves.
Personally I would just go one step farther by installation an original cobrajet 4 bolt main block with early 1970 closed chambered high compression heads for a dominating bit of real good horsepower.
As far as I know it. Replacement of components like this with superior components will actually increase the value.
Like say going from a 351 Windsor engine .....up to a 351 Cleveland engine....or up to a 428 CJ engine ...or installing even an aftermarket Boss 429 engine in a car such as that increases the value if kept within the original racing heritage lineup of engines that were available on cars such as yours....
Like the Ford (429CJ) for your car....It raises the value even if your car didnt come with it originally...others like yours did.
That 72 mustang has alot of room under that hood.
Others please feel free to add or make comments on this subject.
Ray sends...