Your entering the twilight zone!
Know that none of your rocker assembly, pushrods, and so forth can be used that came with the solid lifter engine.
Go to the Engine FAQ and look around. The swap can be done, but you must replace the rocker shaft, rear rocker shaft bolt, rockers, pushrods, and may have to modify the cam itself to install hydraulic lifters. Oiling requirements are greater.
The lifters are oiled via the rocker shaft, rockers, and pushrods on a slant six. There is no oil passage that leads to the lifters through the block. Oil must flow down through the pushrods for the lifters to pump up.
Now that this has been said, I assume you are not building a performance engine. The selection of hydraulic cams is extremely limited for the slant six, unless you have one ground at someplace like OCG, and then they are still pretty mild. If you're planning to pump up the power, you would be better served with a mechanical valve train.
There is a contingent of us that is looking into a mechanical roller cam shaft. These do not exist for all intents and purposes at the moment. OCG is going to build us one based on a pretty nasty Normally Aspirated cam that is noted in the user submitted grinds. Look at the Isky. Probably not what you want if your street driving.
2¢
CJ