I'm in the midst of an engine swap on my 88 using a 2018 Impala LFX with a 6T70.
https://www.fiero.nl/forum/Forum2/HTML/145321.html

I found a shop that converted my Impala axles to fit the Fiero/Cavalier CV joints. Contact Dutchman Axles, Boise, ID (https://dutchmanaxles.com). I'm not being paid by them but am a customer. The cost to modify the axles was $150 plus shipping and three week turnaround including shipping. Hope they can help on your conversion. Mike

The photo shows a stock Impala axles, the Fiero axle and the two modified Impala axles.

THAT looks pretty cool.

The shop did a good job on those axles. To shorten and cut new splines is a decent amount of machine work and the price seems right. The pictures would seem to indicate that the 6T70 w the LFX engine would use identical length swap axles. There are a few of these swaps in progress right now but from what I am reading they are complex and not easy to do.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

An update on the axles. I just assembled them and they fit with no issues. Drove the car for the first time yesterday without issues. Haven't taken any times yet but it has lots of grunt compared to the 2.8.

As for the LFX/6T70 swap, if you checkout my post on doing this swap you will see that it isn't a simple "drop-it-in" swap. But it does fit and allows good access to the alternator & A/C for replacement. The transmission is a beast and there is little extra room on the driver's side of the engine bay. In my '88 the engine needed to be shifted left to clear the shock tower, otherwise you will need to go to coilovers and trim the right side tower about 1.5" or so. I'm still figuring out the electrics too, lots of wires.

[This message has been edited by MikesFirstFiero (edited 10-08-2021).]