Look at the old Hotchkis E-Max Challenger and the Chenoweth Challenger. Both of these ride on a relatively stock K-Member and do VERY well at autocross events. Sadly the E-Max is MIA (I think). The aftermarket K members, and those with coil overs, seem to be just another way for you to part...
A 392 with an 8HP70 auto trans (basically out of a 2020 Challenger) will slide into an ebody without modifications done to the tunnel and using the stock K-member. You will need to use the Holley engine/trans mounts which move the engine forward 1-inch. This gives the transmission some room...
I recently saw a video by the guy that does the barn find photos, where Mopar City in Illinois has several e-body consoles in various colors. You might give them a call.
Did you trim away some of the tabs and bosses on the case that are known to interfere? I've seen plenty of pictures of the areas to trim on the 8HP70 (Attached), but not the HP90.
Download the 70 Challenger factory service manual found over at MyMopar.com. Go over the direction of the centerlink and how all the steering parts connect from a factory standpoint. Then look over the Borgeson box and how it mounts. You may need some help from Borgeson on shimming the box.
Everyone says Legendary is the best, but a good upholstry shop should be able to make most anything work well. Has anyone tried PG Classic seat covers or the leather seat covers by Lseat.com?
1970-1974 Dodge Challenger Custom Real Leather Seat Covers (Front)
Seats
The best way to get the younger crowd into these old rides is the Gen3 Hemi route. Maybe it alienates the stock restoration purist. In my opinion, there's value to be had going the modern engine route. The kids dont know, dont care about old school technology anything, especially carbs.
Numbers matching on a 72-74 e-body means very little. I would almost extend that to numbers matching doesnt mean much on any small block e-body outside of the TA/AAR cars. If you're concerned, remove the engine, tuck it away somewhere. Then find another block to build, hit up Blueprint, or go...
AR Engineering has a custom rear shock plate where you can weld the shock mount to the plate in the location you need it.
Shock Plate, Race Style | AR Engineering
As mentioned if you checked for power back at the socket and have 12v, it may be a ground issue. For extra insurance, I soldered a ground wire to socket bases on my Challenger rather than hoping to achieve the ground from the friction fit housing. I also upgraded the 1157 bulb to a 2357 bulb...
Vendors...Its nice when part prices are clearly marked. I myself and a lot of others never like the parts guy out there that is "displaying their collection" of parts. The reason I say "displaying their collection" is that the part prices are so high, few in their right mind would ever pay...