Adjustable rear trailing arms and caster.
 

I can't seem to find any wheel alignment specs that mention rear caster. I don't know how one would even measure rear caster. I have fitted new rear trailing arms. How do I find out what the correct length is for the trailing arms given I don't have the oem arms to compare? Does it even matter providing both arms are the same length?

Caster is another description of hub pivot inclination, fore and aft. That is described by the vertical line drawn through the upper and lower "ball joints" on double A arm suspension. For multi link it can be hard to determine exactly where the lower pivot point is. I like this link as a show and tell for the suspension geometry, scroll down to the rear suspension.

https://www.edmunds.com/car-reviews/...alkaround.html

Caster is not relevant for non pivoting wheels but the geometry of the connected links is critical. The rear hub caster angle changes with suspension movement so it is the relative arcs that matter. If the trailing arm is longer or shorter than stock then the toe link effective length will be different from stock and deliver different toe rate changes from stock. If you know what you want in that regard you can play with the trailing arm length and an alignment machine if you want to see how it works.

Since the toe link length is connected to both the trailing link and lower control arm through the hub and that length determines the toe setting throughout the suspension arcs you should ensure your new trailing links are the same length as stock. The straight line length from the mounting bolt to the hub bolt is all that matters. Any curves or drops in the arm itself do not affect geometry. Just measure your old ones bolt to bolt and adjust the new ones to the same length, bolt to bolt. Sorry, you don't have your old ones so you'll need to get under a stock car or ask someone to measure theirs for you.

There will be a very slight change in anti squat effect from different length trailing arms also as the arc of the trailing arm influences anti squat angles.

The length of the stock trailing arms is 14.42".
I have been searching for what affects the length of my adjustable SPL trailing arms has on my track car.
On my first outing with all the new suspension bits*, I had the trailing arms extended 3mm more than stock. Initially, the rear was quite loose. With adjustments to the tire pressures and shocks it calmed down a lot, but I still had an unexpected loss of the rear as I lifted to brake.
I have one inch of drop in the rear. My car is really planted with 9k front and 11K rear springs in the rear and weights 2,600 pounds. Very minimal body roll. I also have a splitter, canards and wing.

*Before my mods to all adjustable, all solid suspension pieces, stiffer springs, 3-way JRZ's, huge 275 Hoosiers and Wilwood rear caliper*, it handled quite well.
Any thoughts would be appreciated.

Remove the spring from the shock (or the whole shock) and observe what happens at the hub as it moves through its entire travel. Then decide what it is you want the rear of the car to do, and adjust accordingly.

I could be wrong but I think those SPL trailing arms have a similar impact to that of those traction mods. So it should be getting rid of anti squat and making the rear have more "grip".

I ended up getting the STi trailing arms to go with my MCA traction mod.