283 oil flow problem

I'm stumped on this one. A little history, so bare with me...
I have a pair of 1962 283's in a chris craft boat. Both are totally rebuilt (1 reverse rotation), with the block and head work done by a highly rated local shop. The only real similarity between these and one in a car are the heads and blocks. Everything else is different. Both were built using identical sets of parts (cam and timing gear/chain are different), lifters, pistons, push rods, rockers, oil pump, etc.
The std rotation one (timing chain, vs gear) was finished about a year ago, and run for only about 30 minutes. I used the actual gauges and senders, but only verified they worked. Could have been off. At that time I did register oil pressue with the old gauge, but don't remember what it was. After an unfortunate series of screw-ups on the transmission rotation (whole other saga...), I had to pull the engine out of the boat to change the trans. Once I had that straightened out I ran it again. This time with brand new gauges.
Initially the gauge registered about 50lbs, but a problem with the water pump caused us to loose sight of the oil pressure for a while. Once the water pump was resolved, and another trans swap (continuing saga...), all appeared fine until we realized there was no oil pressure. A brief rev of the engine only caused a slight increase in pressure.
After verifying all the obvious things (sender, guage and oil level), I pulled the tach drive (distributor is relocated to other end in special casting) to access the pump shaft. Spinning with a drill still only gets to maybe 8lbs. (got 35 this way on other engine)
Since it has a remote oil filter, I pulled the hoses and spun it again. Much to my shock, oil flows from BOTH hoses. I pulled the 'filter bypass plate' off the block and spun the pump again, and the same result. The oil flows from both the pump port, as well as the port feeding the block.
I've searched the internet for days, and have found little help. One engine shop I had access to said 'wow I've never seen that happen'. My shop is only accessable in person, so I've not yet had a chance to talk to them.
What I have learned is that there are/may be various bypass and relief valves in the system. Another post on this site had some good info and pictures, but just does not help. It seems that there might be an internal bypass in case the filter gets clogged, and most pumps (I have std melling sbc pumps) have an internal pressure relief valve to regulate the high pressure.
Can anyone help clarify exactly what I can expect to find in a 1962 SBC 283 in this regard (casting number is 3789935)?
Has anyone had this problem?
Any thoughts or suggestions would be appreciated!
FYI, this is a near showroom restoration project, and tearing down the motor is an absolute last resort. Not to mention the fact that the timing cover casting (very LARGE) can't come off until the trans damper flange is removed (with a oxy/act torch heating it to 800 degrees, and a puller). There is an access plate on the oil pan, so I'm hopeful I can pull the oil pump if need be.
Thanks!

http://www.sdpc2000.com/faq/kbhowto.asp?mt_node=4

maybe?

Robot,
Thanks!
I did read something about a screw in plug in this location in some other post. The suggestion was to probe down from above and compare the depths on a know good to the unknown. Sounds like a good place to start.
Regards

Looks like this may be the issue, but the drawing in the link is very different from my block.
Looking down the sender tube there is clearly the outline of 2 mating surfaces about 8 inches down. This puts that point at about the centerline of the crank main bearing cap, but it's not a dead end, as the picture suggests. The port to the pump? Can't tell much more about what is going on by looking down.
I then pushed a wire into the 2 ports in the filter mount area, and both stop after only about 2 inches, and sure enough they both are visible in the sender tube. Cleary there is no plug, and this would certainly explain the problem I've got.
Now, how do I get it resolved?? As I noted already, taking it apart is not a simple task. The pan is bolted through from inside the timing casting, and that damn heat and press flange totally blocks access to the bolts.
I'm thinking - put it in from the top. It's a clear straight shot down, not much different from pushing it up, just further. The real challenge is - how do I know exactly where to put it??

We also have a 1964 Chris Craft with twin 283's. I enjoyed your description of our rebuilding project. Sorry I do not have an explanation or advice on your oil and oil pump mystery. I do have a question on the water pump. We have a overheating left motor and are not getting good water flow through that motor. Disconnecting the pipe from the water pump to the motor gives us good flow. It seems that the pressure is too low. What should the pressure be on the water pump? Any other advice will be appreciated.

Regards,