Ouch ...... double ouch!
Those blocks are definitely thinner than the '75 cast #481988 block I have here.
Pulled the block out of the bag (looking a little rusty) and snapped off a few images. Notice this particular block was cast late '74 but has the '75 casting mark.
Most importantly, look at the same main web (#3) as shown above. Much thicker in the early '75 block. Straight up from main saddle to cam tunnel essentially.