Mine cost me 6k$. But I didn't cheap out on anything. Brand new act clutch, torque solutions motor trans and pitch stop mount. We're 1100$ alone. Also did the custom driveshaft like the op and hammer down. Costing around 650$
If u got a stock clutch with your 6 speed and are stock power and don't want to replace it. Along with find a used stock trans mount and auto driveshaft. U could Probaly do the swap for around 4500$ or so. Goodluck finding used spec b rear axles tho. I got lucky and bought a swap coming out of a legacy. Got teansmission and rear spec b axles for 3300$ 06 sti 6 speed with 24k miles.
And a rear diff for 425$