3.9.21 Edited to remove brainfart......
The later 289 engine block and 302 engine blocks have the same bolt pattern. What differs is the ability to carry a larger diameter flywheel and pressure plate assembly. This being the later casting, should take at least a 10.5" clutch assembly and "maybe" an 11" disc. The pressure plate is what can interfere with the bellhousing when you go larger.
At this point, you would come to the decision based upon at what amount you can purchase a GT40 bellhousing. This bellhousing you have is a late production 302 version. How many cars had this piece???? Unknown. You would need other members to chime in on the way that their cars are built.
It may be easier simply to keep this piece and use it....... (did I say that!????) Again, I have no idea how easy it is to find a SBF bellhousing for a GT40 these days.
Anyone else out there with a single arm (LH) bellhousing that can chime in with a serial number for reference?