Gary, try to mill close to the angle of the original carb mount, however you will have to weld or braze in all the low areas on the log or you will have some gaps for the adapter to seal on the log.
I would purchase one of Mikes heads rather than do all the machine work necessary & still be 40+ hp short, compared to the new aluminium head.
You will have at least $600.00+ in the old head & still will not even match the new head bone stock. Your choice??
If you can do all the machine work yourself then the cost would be just for parts.