Well as an old hippie and veteran from back in the 70's thats a good question. I was with an aviation electronic warfare unit and had flights all over the world on a regular basis with no customs and back then no dogs or drug tests. I don't think the smoke today is any stronger per se, most strains have been watered down (mixed between indicas and sativas). It is hard to get that soaring, laughing, giggling, freak'n high when you have indica mixed in everything. Today we have selective breeding and yes make bigger yields, more resin on the buds but a much more IMHO blended high today, than in years gone by.
I think the other big difference today than in the 70's is precartel and big business. The root of all evil, the search for the almighty dollar ruined the old strains mostly in the search of more weight and shorter flowering times. Remember the old landraces take 16 to 20 weeks easily when two crops can be grown in that time with most of today's flowertimes.
So I don't think there is much difference overall in strength I believe it is more about the type of high today over the 70's. Best example it is like drinking single malt scotch all you life and then having to drink blended scotch, does it get you drunk yeah but just not the same.
Happy Growing