I see a few clear routes at this point:
OPTION 1: OE-fitment custom spring / OE-style shock
This requires a custom spring which fits the stock upper and lower spring perches.
SHOCK OPTIONS:
* Revalved Koni Red single-adjustables (note that these were discontinued by Koni, and are not user-rebuildable) -- rebuilds are expensive ($120+)
* Bilstein AK series shocks -- direct fit available, but need to be revalved by Bilstein. Non-adjustable. Can potentially be converted to take-aparts and revalved by the end-user with standard Bilstein race parts. 46 mm models may require a modified lower control arm or bottom mount spacers.
SPRING OPTIONS:
* Any spring rates (fully custom)
PROS:
* Relatively inexpensive
* Minimal (or no) fabrication required
* Any spring rate can be selected
* Minimal shock cost ($75 each for Bilstein AK series, plus $75 each to revalve them, for $300 total)
CONS:
* Custom springs are more expensive than off-the-shelf springs (~$250+/pair for custom vs $150/pair for high end off-the-shelf springs)
* Shocks are not rebuildable or revalvable by the end user, except in an unsupported configuration (AK series converted to take-apart)
* No adjustable shock options except the discontinued Konis
* Not possible to adjust ride height / weight jacking without changing springs or adding/removing spacers
* Small non-motorsports shocks like the Konis may not be able to handle higher spring rates. The 46mm Bilsteins should work fine but require more work to fit.
TOTAL COST:
$550+
OPTION 2: QA1 Tapered Mustang Spring, Bilstein Motorsports Shock
This requires significant modification of the lower control arm.
SHOCK OPTIONS:
* Bilstein ASN series coilover shocks -- aluminum-bodied race shocks that are user-revalvable and rebuildable, with all parts available individually. Easily converted to use remote reservoirs. Can adjust gas charge pressure. ~$220 each + spring perches.
* QA1 adjustable coilover motorsports shocks for the Mustang II -- these don't have the same reputation as Bilsteins, but they are adjustable, and QA1 can revalve them and provide dyno charts. ~$180 each.
SPRING OPTIONS:
* 375 lb/in (too soft...)
* 500 lb/in (decent upgrade from the 400 lb/in lowering springs)
* 600 lb/in (getting better)
* 700 lb/in ("good enough")
* Custom (anything)
PROS:
* The shocks can handle pretty much any spring rates
* Adjustable shocks or user-revalvable shocks
* Shocks are designed for motorsports use
* Requires no modifications to the upper spring perch
* Adjustable ride height / weight jacking
* Inexpensive off-the-shelf springs ($150/pair)
* Can use custom springs if desired ($250+/pair)
CONS:
* Top mount modification may be necessary for the Bilsteins to fit, if there isn't enough thread on the end of the shaft to allow the Fiero's stock bushing-on-pin arrangement to be used. The QA1s look like they fit with minimal modification perhaps just by increasing the hole size if anything at all.
* Significant modification of lower control arm is required
* May affect wheel clearance (may need larger diameter wheels to clear the bottom shock mount)
* Very limited selection of spring rates unless custom springs are used
* Shocks are very expensive
TOTAL COST:
~$720 for 700lb/in springs, Bilstein ASN shocks (p/n B46-ATAD4), and spring seats (p/n 192190). The valving kit (p/n 192153) is an additional $300 and there are some basic tools required as well. I don't know what valving the shocks come with, if any.
A shock dyno is needed to develop valving. It's possible to
make your own for $200 to $400. A commercial unit is $2000 to $8000.
I will update pricing information (valve kit, spring seats) when I find it.