I had that issue with my 4l80e, I wanted an adapter fitting (trans thread to -6an) but could not find one, Vince at FLT trannys makes a custom one, but only for tranny customers...so I left the stock tubing/fittings coming out of tranny, cut about 8 inches back, used some compression to -an adapters and ran my braided line to my tranny cooler up front. Looks good to me.
I know there are 4l60e -an adapter fittings out there, I have seen them on several builds on here and ls1tech.
good luck.