[dsraven]

if the springs are brand new they will, of course, sag some. what you wanna end up with is the lower control arms to be level with the ground at ride height.
sounds like what you are gonna be looking at is the firewall modification. wht I recommend to do is first cross brace the area, behind the firewall, in such as way as to support the area but not be in the way. the cut away the area in question. do your engine install and leave room for the oil pan and pulley/fan set up between the rad. then when all that is in and you have that completed, worry about the firewall. you could then fab something from scratch, use a modified steel wheel barrow tub or buy a recessed firewall kit.
without seeing your truck, I highly recommend to check the hinge pillars and cab mount areas before going too much further. sometimes the firewall is whats holding things together.