I have seen some aftermarket tabs. On mine when I bought it,
they had a cover off a 68 or 68 340 which ever on had the marks on
the passenger side of the cover. Anyway, I fixed it by putting a line
at tdc on the cover with a thin black marker and used that, it can
get you close. I also added some timing tape to the damper.
When I did a tear down I changed the timing cover but the above
worked for a year pretty well and it was inexpensive.