Oh flip, i forgot to put the standard V8 back after sorting the clipping issue. i have put out a correction for the engine and realigned the downpipes and exhaust that i had forgot to move. i am stumped as to why the suspension and tyres are playing up. If there was anything wrong in the config i would expect it to show up on my end too.

No it's not a mod conflict. After I removed the other version I stripped the car aggain and left the game. Now when I entered the game aggain the car had V8 DOHC Supercharged installed in it!!! No suspension and tyres this time, only engine and exaust.
But hey, it had an Air Filter! :)

I know, but i checked this mod and the V8 Super clipped on this one too. The conflict could be down to how the other person completed their upload_config although i am stumped as to how it would happen.

The clipping V8 Supercharged issue I reported was on the '71 Cutlass, not this one. I haven't put V8 Supercharged in this one yet.
About the parts reapearing I found that I have installed another version of this car - an Ouroborus mod or something(I don't remmember when ot how), and that may have caused a mod conflict. Not 100% sure but since I've removed it and will test it further.

I have issued an update for the missing air filter and to correct clipping issue with the V8 Supercharged engine. i genuinely do not know why the stock engine would reappear or the other issues with tyres would occur.

Last night I stripped this car of everything, repaired all parts and bought what was missing. All the parts for the car, including for the engine were in my inventory when I saved the game and exited. Now when I log in I found the car with stock engine mounted, rear axel is fully installed and 4 tires mounted, even if there is no front suspenssion in the car - front tyres are in the air, one of the back tyres is a drag tyre. All the parts that were in my inventory are still there! I can provide screenshots if needed.
I also modified the '71 Cutlas and there is no problem there but need to mention that car was in the parking, not in the garage. I mention this because I saw in the comments of the '71 that people are having simmilar issues.
Mods I'm using are 3 of your restomods, 2 Fastback's and 2 from Nexus, I can provide list if needed.
P.S. Please fix the mods I really like them! :)

Not intended, will add it in and issue an update asap.

The car is missing an Air Filer, don't know if it's intended ot nor!