well......I'm sure this question has probably been asked alot, but here goes nothing....
I have the chance to get an 88 N/A RX-7 (i think to guy said it was a GLX) with a blown motor. If I bought a Twin-turbo 20b Mazda Cosmo half cut, what would I have to do to swap the 20B into the rex?

 

Quite a bit of work. Quite a bit of money.
Use the search function, or just browse through the 20B forum. You should be able to find out all the answers to what you need to know.

Just for your information...
20B -> FC3S swaps generally cost about $10,000 - $35,000 (american) depending on what modifications you do (single turbo? high boost? drivetrain?), what car you are starting with, and if you pay someone to do the work or not.
You will definitely need a complete TII drivetrain (tranny, driveshaft, rear end assembly) which would be fine with a stock-turbod 20B. If you want to have an upgraded 20B, then you will need a transmission swap (T56, Supra Box, Custom unit?)

Lots of work and lots of reading to do.
Just do a search and read up what you can.

 

That would probably be a GXL, which is the "luxury" model, or maybe the GX, which is a Canadian base model.

I have three suggestions:

1) Start with a TII.
2) Start with a TII.
3) Start with a TII.

Note that these suggestions are coming from an idiot who started with a non-turbo RX-7 for his 20B project. It is always better to learn from the mistakes of others as opposed to learning from your own mistakes.

Yes, plan on $10K to $35K USD for the project. Mine is going to end up closer to $40-50K USD, and an all-out SPO racer is going to cost around $50-150K USD.