If it is slipping in forward gears, it's usually the forward clutch plates are worn out- they are the weakest part of this automatic transmission. To fix them the motor must be pulled out, but the plates can then be replaced without taking the motor off the gearbox.
If you are talking about the brake bands, they can be adjusted in the car, once the front cover is removed. However, they do not normally cause transmission slip.
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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
