The other day I bought a set of H&R coilovers for a 350z from someone on fbm for a measly $200, a lot cheaper than their typical price. The seller was nice and the suspension was good, but one of my OE shocks wouldn't come off so I was left with one side with the new spring but the aftermarket shock (They weren't the original H&R shocks either, they were KYB Excel-G shocks), and one side with the new spring and the OE shock. When I took the car off jack stands, the front and rear were at completely different preloads and my car was effectively squatted. Me being lazy, didn't feel like fixing this until the day after (It was 2 am on a school night by the time I finished). The next day, the spring with the aftermarket shock broke in half right after the dead coils.
TLDR: I'm stupid and left my car with uneven preload, leading to a very unhappy spring.
The next day I put back on the OE rear suspension, springs and shocks, and lowered the preload of the front a little bit because it was super stiff when I installed it (lowered it by about 9 threads). I am still experiencing a lot more bump steer and a rougher ride though.

My questions for people smarter than me: Where do you think I could get a singular or both brand new rear springs (part 29237-1)? Should I just bite the bullet and go back to full OE suspension? Am I fine to drive with the aftermarket front, if so, what preload should I keep it at? Will there be any significant long term effects on my car that I should assess?

I am not a fan of paying a mechanic more money than I want to to fix something that I can do myself with the right tools, I just need to know how to be safe about using aftermarket suspension because I daily drive my car and can't deal with a ton of issues.

 

If i can buy a decent coilover kit for my 07 G35 SEDAN for less than $400, i'm sure for a 03 Coupe will be half that on gBay or Scamazon..
Eiterway, anything aftermarket will stiffen your car, your back, spill drinks, bend rims, drop nuts & bolts randomly.. A great coilover kit is well over a grand but i haven't had a coilover kit that would come close to OEM ride of it's softest setting..