BIGRIGZ
+1y
I think personally, that if you cut the back wall kinda low so that it's like 3-5"s above the tranny hump on the rear wall that looks the best plus, then you can just overlap the back cab wall instead of trying to mate up sheetmetal.
In the firewall I think as far up on the vertical as you can make a line to cut then mark your second/low side line (however many inches you're bd'ing, 3,3.5,4"s) you'll have to put some new sheetmetal in the front floorboards.
on the sides try to cut it so that you can get the longest line from then back cab to the firewall, remember tho that you'll have to make a little zig/zag around that front body mount kinda by the kick panels.