The tile pretty much says it, what all is required to convert to a two piece rear main seal crank from 1 piece?

My understanding is that I will have to:

Get a seal converter/assembly (bolt on, no machining required?)

Get a neutral balanced flywheel/flex-plate to replace the one piece one I currently have.

And thats it? That seems too easy...

Why would you want to downgrade to the two piece main seal?

most high end cranks are designed for a 2 piece, so to run the crank I want I need to

It also lets you run a neutral balance flywheel

Check with the maker ;
some adapters will take the 1 pce pan , some require a 2 pce pan

ok good to know

The adapter is in the center:

I have used a lot of the later one piece rear seal blocks and used a 2 piece rear seal crank.

As mentioned you can get the rear seal adaptor for either for either the early pan or the late pan, I prefer the late pan as you can use the one piece oil pan gasket from GM = no leaks.

Also you will need an alignment mandrel to locate the seal adaptor.

so im contemplating putting my L.J. 68 steel 327 crank in a 1 piece block to get the added benefits of roller cams and what not. In your opinion, would i be better off doing a bit of work to say a late 70s 4 bolt to accept the spider and lifter holders ? Also, where did you get that shmansy looking adapter contraptor in the picture.
a slightly confused mind wants to know. 😁

You want the later 1-piece rear seal block because the machining is much more accurate than any 70's block.
Not that a 70's block can't be corrected by a very competent machine shop, but you will pay dearly for that.
The better choice then is a DART block.