w boxes are priced between $350 and $650 dollars. this is fact. i can have one landed on your door for less than $500 easily.
for the conversion you will need
bell - CRS, dellow, nite parts, ect ect. or the adapter kit for azcustoms. $600 max.
a new spigot bearing? $10
your clutch modified to accept the input shaft. $100 max.
a modified tail shaft, or possibly just the front section from a celica. $300 or less.
a custom gearbox x member. this is the trickiest part and it is not even hard.
the part is not heavily stress or a complex shape.
it is basically 2 pieces of flat bar with 2 holes each for the chassis tabs.
one piece of say 40x40 or 40x20 rhs. with 2 hole saw holes and 2 20mm holes in it for the bolts to pass through/sit inside the section.
any fabrication joint can do this for you. even an exhaust shop. you could have everything else done and have you car towed to a workshop, have this part made, be charged between $150 and $300, and drive home.
this is all kinda "worse case scenario" pricing.