I moved the seat back on both my 2nd and 3rd Gen 4Runners. They both are similar. The seat mounting tabs are riveted AND welded to the seat track. Drilling out the rivets is pretty straightforward, releasing the welds without hosing up the brackets and/or the seat track is the tricky part. On my 2nd Gen I used a chisel and hammer wedged in between the bracket and track to free the tack weld. Lots of trauma to the assembly from all the hammering. On my 3rd Gen I drilled out the welds. The additional drilled hole was used for bolting on the angle iron (actually channeled steel) for repositioning the seat rearward.

If you have basic mechanical skills it is fairly easy to do. Take the seat out (4 bolts) and you will be able to see what is involved.

I don't recommend extending the seat more than 2"s. The shoulder belt starts to become ineffective and the outside mirrors are at their limit of adjustability.