This problem is a little different from the common fluid burp problem.
With the common problem the fluid will burp out of the filler tube after the engine is started after not being run for a couple of weeks. This iscaused by the fluid in the top half of the torque converter draining down and overfilling the sump. Then when the engine is started the fluid gets churned up and will burp from the filler tube. This problem can be prevented by having the lever in NEUTRAL as soon as the engine is running and not turning off the engine for about three minutes. This gives the converter a chance to refill quickly. If the lever is kept in PARK the converter will NOT refill and the fluid will burp out.
A 1950-54 Power Glide should not have the engine running while in PARK for more than a short time as there is no fluid being pumped theu the transmission. This shuts off all internal lubrication also.
Was anything done with the Power Glide while the engine was removed? The most common cause of your short term burping would be a seal leat at the oil pick-up pipe.
First have the fluid level between add and full with hot fluid. Check the level with the engine running and lever in neutral. Then drive a bit and make a stop. With the engine off check level. After a few minutes (or more) check the fluid level again. Did it rise? It should not.
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Chevgene