Well from a quick read through the quote that is literally every-thing or pretty damn close to it. You are going from an auto/column car to a full manual trans and need everything it looks like. If you get it all from them it's gonna cost more than going from a 4 speed to a 5 speed in an existing manual car. You even have hydraulic clutch set-up sourced there at $629, plus pedals, flywheel, flywheel bolts, clutch, shifter, shifter ball, shifter ball nut, shifter boot, shifter boot chrome hold down, and on and on, literally everything including many parts you would be replacing in the resto if you had a 4 speed car and were just restoring it as is.
If a 4 speed guy were just replacing all his 4 speed stuff that typically needs done on this list it would be pushing $1.5-$2K so it's not really that bad when you think of it that way. And the manual trans experience if you keep it is gonna be priceless. So you could look at it that way or like you said, sell this car and buy one that's already a 4 speed but needing most stuff refurbed and speed a good deal of money too and still have a 4 speed car. Of course having an actual 4 speed car is a plus and you could have fun with that while you are thinking about spending $4K to upgrade to a 5 speed.