The guys have you covered pretty well. A couple things I'll add in the order of your questions: No the shift linkage is not the same. The 205 has a basic "stick" that pivots on the One bolt that holds it onto the adapter/mount. The 203 has a "shifter "mechanisms" with Two linkage rods that hangs off of the side of the case.
Your driveshafts will be too short due to the additional thickness of the 203. Your crossmember will likely need altered slightly where the mount sits as the "footprint" of the mount differs...One hole needs slotted an 1" or so.
As far as the adapter, it depends on what transmission you have & what the 205 is out of. I'll ASSUME you have a TH350 since that would be most likely what would be in a "77 in front of a 203. If the 205 is out of a TH350 truck (26 spline)...your all set if the mount/adapter is still bolted to the case when you get it. The TH350 205 is somewhat to downright rare compared to the number of 203's out there. The 205 behind a SM465 (16 spline) is everywhere & then yes, you need an adapter that Magic mentioned...it goes from 26 spline input to 16 spline output, as also mentioned...it's pricey & you would be better off looking/paying for a TH350 unit. There's my free Two cents, Lorne