The 164 front springs are stiffer and longer than 140 springs,
or so I've been told. You probably don't want to mess with
those in the front.

You can get stiffer springs from salvage yard 240s. The wagon
springs are the most stiff. Sedan springs are more stiff than
the stock 140/164 springs. It doesn't really matter, though
since the stock springs are likely completely smashed to
hell. The ones in my 164 looked like question marks when they
came out.

If you swap from 240->140 grab the top mount plate from the
240 -- it's shaped like an upside-down bowler hat and the springs
from 240s need that (140s are held captive with a retainer on both
ends, on 240s only on the bottom).

I found some wacky progressive rate springs in a 242 turbo and
put those in my 164. It's nice and plush but there isn't much
roll in the back even with KYB shocks in the rear.

I think the guy at v-performance.com has progressives for the
140 front springs but not for the 164. You can get new 240
springs in a wide range of spring rates from lots of places, just
grab a mounting plate from a 240 at wrecker.
chris