I just bought a 77 21 centurion cruiser 460 ford and a berkely. It has forward and neutral only and yes i have checked to see if the reverse bucket closes all the way. The pump is set further back then stock it has a droop snoot, 4 deg wedge installed with the thicker end up (down) place divertor, ride plate and shoe. The guy i bought it from even made a custom rams horn type exit from the reverse bucket, so it has 3 ways for water to get out. Any ideas?
A bit difficult from the photo to see but suspect you will find that's a AT "Turbine trim" (*vs Place Diverter, similar in design but different) You may have already tried experimenting with raising the nozzle up while slow speed maneuvering (as if to use "up trim") to effect better more effective reverse flow , likely is, looking at the picture the reverse thrust is backwashing partially into the ride plate area versus forcing water under the transom , and agree an OEM reverse bucket would help I suspect , have lots of nos buckets Im sure, call me if we can help Tom
* additionally suggest you put a straight edge on the keel and check the ride plate adjustment with a level and angle finder , looks like its set "low" from the picture, there may be a reason for it considering the added trim tabs, may be fighting or trying to control a porposing issue ..