As you already know ive done this build up, Still doing it. I have a spare motor for my l31 build which im happy to part with the pistons and rods if need be. Also got another set of n/a pistons. (l28e gear im talking about)
If you need a spare rod or a set pm me. And we can work something out.
When it comes to putting the l28 into the hr30. You need to modifie the oil pick up, There is another boss towards the front of the block for the MAXIMA setup and the south african r30's, This needs to be drilled out and threaded, then you move the oil pick up to there. Block the old one off. Install the l20et sump and your running.