I used Global West for rear springs, they offer stock, lowered, and an even lower inverse rear eye. Not sure who mfg them though.

As far as height change I really can't say, the ones I got were the lowered and are supposed to be about 1 1/2" lower, but since I put them on a car that I didn't check the original ht I can't tell you what it did in comparison. I then added another 1" of lowering blocks to the axle rather than get the inverse eye.

The Tech guys as GW are pretty good, just ask for someone that knows early F body springs and heights. Hotchkis Tech should be able to do the same.
You can also go to the Pro Tour forum, almost all FGF guys there have changed springs at some point, but mostly to lower a car.

BTW, I really like that 350 HO look, not something you see much. I wouldn't want to change the ht much, just maybe firm it up if it's mushy.