Transmission lines
 

I have a place in my transmission cooler lines (hard lines) that had to be cut for clearance for my motor swap (lt1). I had it just made with a flex high pressure rubber line, but I had lots of problems with leaking. Can I just use some adapters and use copper line to mend this piece back together, using compression fittings?

Compression fittings work fine, we cut mine and put them back together with compression fittings. Just don't over-tighten them.

I wont have any adverse reactions from the copper, right? I really doubt it, i just like to be positive.

It will be fine.