I have a 70 Chevelle with a 454... What's the best way to drop the front end - with springs or drop spindles?

springs are the way to go but it depends on how much drop you want, you may need both

Dropped spindles will move the bottom of the wheel (rim) closer to the ball joint, which should be checked for clearance. Not often a problem here, but it does come up. Remember, a "dropped" spindle is in reality a raised up axle height.
Spindles vary greatly in quality, not just of machine quality but importantly the quality of the steel or aluminum alloy used in the mfg of the part. There are some scary spindles out there, but you might never know which ones unless you ask the right people.

Shortening an existing spring does not necessarily have to be a bad thing, so long as you do not destroy the temper of the spring steel in doing so (cutting coils).
The real issue here is in reducing the number of coils, which will change the characteristics of the spring. Spring harmonics (same as for valve springs) are affected greatly by the number of coils in a spring. For the same wire diameter and metal temper, the more the coils the softer the spring, as a general rule of thumb, but even here in an automotive suspension application there are other considerations to apply when designing the coil count.
Do not heat them, as most everyone knows these days.

Buying "lowering" springs is not a free ride. You must realize that you are now entering issues of wire diameter, coil count, and metal chemistry. Gotta do your homework here, which ideally would include going for a ride or driving a car with the springs you are interested in.
Long spring with more coils but large wire diameter, or short spring with fewer coils and large wire diameter, or short springs with fewer coils but smaller wire diameter, or ..... or ....

When it comes to springs, you often find surprising results you'd never expect. Dick Guldstrand's 700 lb / inch coils for road-race Camaros are totally different in feel than a custom set of 700 lb / inch coils I had made.... same height, same rating, but different wire diameter and number of coils. Guldstrand's 700 pound springs lowered my canyon racer Camaro 2 inches and were stiff enough to crack the subframe in 3 places, where the custom springs, same rating, same lowering amount, had a very nice and controlled ride, night and day difference, yet the same rating......

Good luck !

Eric