your take on BOV's and WG's

You need a waste gate to control the level of boost from a turbo period. If the turbo only produced the desired boost at max rpm it would produce little or no boost below max rpm. You size the turbo to produce the desired boost low in the rpm range then use the waste gate to prevent over boost higher in the rpm range.

A Blow Off Valve is used when you have the throttle in between the output of the turbo compressor and the engine. When you close the throttle of a blow through system it creates a pressure spike that travels back into the turbo compressor and stops the compressor from spinning and rattles the bearings. The BOV releaves this pressure spike which allows the turbo to keep spinning and protects the bearings from Damage. The reason you want the turbo to keep spinning is it reduces turbo lag when you open the throttle again.

I don't know anything about the Cummins turbo but for all around driveability and perfromance it is important to size the turbo correctly. One from a 3.8L (231 cid) Buick V6 would likely be a better match to your 273 than the Cummins turbo.