Just lay the seal in the hole flat, lay a 2x4 on the seal and tap in nice n easy.
Oh you may have got a later style seal. I believe in 69 or 70 they changed the timing cover so that you could change the seal without needing to take the timing cover off. Earlier ones had the seal tapped in from the back with the cover removed.
The timing cover seal really doesn't want to go in, it's kinda thin walled and I was concerned that I may be hitting it too hard and it cuts into the 2x4 like a knife. it presses in from the back, the flat side should be down right (Hollow side towards back of engine)?
As bad440 said, thats the right way, just be patient. I learned to always have a spare on hand...if yer buying one, 2 wont break the bank. Like Happy Gilmore...."just tap it in, tap tap tap"