904 tranny problems

My 904 was rebuilt about 5,000 miles ago but since then restored my car and did an engine swap to a 408 from Blue Print engines and changed torque converter to a ATI set up for the street not the strip. Engine produces 500 ft. ft. lbs. of torque.
I have 2000 miles on the car since it's been back on the road and noticed it started slipping into third gear when giving it a bout half throttle.
Was told the bands needed to be adjusted so I did and helped. I adjusted both bands at 72 inch lbs. then backed the adjusting bolt on the forward ban 1 1/2 turns and the rear band
2 3/4 turn. After 40 miles the tranny was slipping in reverse so headed for home and noticed starting out from a stop it didn't want to go in 3rd gear but it tried to but would just slip
1st and 2nd gear worked fine. When I had the pan off I changed the filter and fluid and all was clean some sediment on the filter but nothing bad and oil was clean.
I'm no transmission guy just trying to learn and I know the tranny can't handle that much torque from what I've been told but not racing it just a road car that I like to stomp on the gas now and then.