Problems Putting in the selector shaft seal

Many years ago a old school service tech I worked with explained to me, "sure, If you peel the carpet back and drill through the floor pan in the right spot you might knock that seal in with a socket and extension but... You would need a helper under the car to guide and control since you can't see what is happening underneath. You'll work closer to the dash than you might imagine also which restricts drill access and your hammer swing/blow."
So I'll just suggest you sight a line and punch mark where your hole should be from underneath, Then look inside at where you would work.
fast forward to about 4 years ago, That selector shaft seal started leaking in my current 67 notch. I dropped exhaust pipes from manifolds, removed dist' cap ( 273 ) and trans cross member section/support so I could tilt the trans as far as upper radiator hose and motor supports would allow. Successfully replaced that seal. All good? Nope, Approx' 4 months later the front trans seal began to leak. Stands to reason since all these shaft seals are the same age. That seal still leaks today, and a hellofa lot more than the selector shaft seal did. :( Good luck with yours