Forget about messing around with GBox's IMO. If you're going to get a GBox get a B18C7 or JDM B18C box. Isn't such a big deal for what you want out of the car.

For $4000 try to get a B18C2 full conversion with the standard GBox included (does the job fine seriously). Might pick it up for around $2500 or so, put aside $500 for mounts, another $1000 for labour minimum, and then consider that you might need another $500-1000 for fluids/other issues that arise.