Last year I picked up a '67 Lemans, an original 6-cylinder car. When I got it it had the original 6-cylinder springs. A much heavier 400 was swapped in about 3 years ago. After that time, the front end was about 2.5" too low and seemed to bottom out on every small bump. After new PY GTO springs, shocks, and total PY poly front end rebuild all is 500 times better. So far this and the front disc brake swap were the most noticable and satisfying upgrades I have done. Highly recommended!
Per my experience the weaker springs will not hold up.