You did not say what gear is slipping or when. If the cooler lines are on the headers the trans fluid can overheat, thin out and cause the trans to slip. Move the cooler lines away from the headers as far as possible.
Check the fluid for a burnt smell and dark fluid. You could could drop the transmission oil pan, pull the filter and look for metal and/or clutch lining material (looks like small dark flakes of thick paper) in the pan. You can adjust the bands while the pan is off with a short 5/16" wrench. Loosen the lock nut about 3 turns and with a short 5/16 open end wrench tighten the adjustment screw good and tight then back off the 1 1/5 turns then tighten the lock nut while holding the adj. stud to hold the setting.
There will be two of these band adjustments then install a new filter, clean the pan up and install. In 1972 it should still have a drain plug in the torque converter. Pull the little inspection cover on the front of the bell housing and bump the engine over until you see the little 5/16 pipe plug and drain the torque converter. When you put that all back together you can put in 5 quarts of trans fluid and start the car. Block the wheels and put the shifter through all the gear positions then put in 2 more quarts of fluid run the gears again and put shifter in NEUTRAL POSITION and check the fluid level. Keep adding fluid until it reads at the "ADD" mark (this is full with a cold transmission). Then take it out for a test drive. Your 904 should take around 7-8 quarts all together.
A quick note if you can find it use Type F transmission fluid in your 904 or any transmission that doesn't have a modulated lock up clutch in the torque converter (Ford C5, AOD, GM 2004r, 700r4, 4L60, but not the 4L60E, can use it). It will help the clutches grab better and is cheaper than B&M Quick Shift that has more of the same friction enhancers in it than Type F. You can still find it at Oreilly Auto Parts or you can special order it.