A delay into reverse in early high mileage cars with 722.3, 4 and 5 transmission is a common problem, clutch Flaking , excessive clearance is the cause Did you check the fluid level, if full, I would remove the pan and filter, check for clutch and band debris in the filter and pan. If it’s clean, change the filter and fluid, adjusting the modulator two turns CW may solve the slipping problem.