Hi XDraconian, That is a nice piece of model
! Indeed, 430k is far too heavy, but I think you should be able to bring this one down by quite a lot. For the back seat and co, I would go for more details on that model (not directly used so no poly constraints) and then when you feel it has enough details and like your model, I would start the shrinking process (it would be a shame to leave all this work unfinished on the high poly). Depending on the method you used to build this high detail model, it might be easier to start of a blank plane and remodel the stuf on top of this model.
As a blender user, I would also advise you to use the sharp edges flag with the edge split modifier. I joined you an example of my latest work with the indications where to find the options:
The cyan lines are the edges I flagged, as you can see it works quite well to control smoothing. This might help you with your car for the sharper lines. If you also have the budget, you can bevel the edges with 2 subdivisions and mark both extremes of the bevel with sharp lines, this simulates quite well the small radii. For the interior, you could also redraw the shapes with minimum details and bake the high poly details onto a texture. On the interior model focus more on the front door panels, dashboard, the steering wheel and the visible stuff.
If you need anything during the creation / exportation process, feel free to ask me. Also if you need help with physics for this car tell me.
Looking at you model, I keep wondering what it would be like to have a race version as a bonus with big fat good years, a rollcage, 800 hp and side pipes. In that direction
What do you think ?
Keep up the epic work, it is going to pay dividends when finished.