Funny this popped up now.. I did my first 518 install about 25 years ago, two weeks ago I got the car back, the O/D still works great, and trust me it's good that it does, the car has 4:30 gears... But in O/D it acts like 2.94's....
I posted the photos of how I'd trimmed excess material off the trans & fabricated a crossmember once about 22 years ago and since then those photos have popped up probably 50+ times in posts I was reading. At one point US Cartool posted about a project they were getting ready to cut the floor and crossmember out of to allow a 518 to be installed.... I PMed him and sent pictures.... Eventually they wound up marketing the crossmember/mount... Still being sold on their website...
