Yes, if you move the battery to the trunk, there needs to be a metal firewall (includes package tray area), unless the battery is in a sealed (NHRA Approved) battery box. Also the battery hold downs should be attached to tabs off the frame rail or through the frame rail, not just through the trunk floor.
Quote:
8:1 BATTERIES
All batteries must be securely mounted; must be of sufficient capacity to start vehicle at any time. Batteries may not be relocated into the driver or passenger compartments. Rear firewall of .024-inch steel or .032-inch aluminum (including package tray) required when battery is relocated in trunk. In lieu of rear firewall, battery may be located in a sealed .024-inch steel, .032-inch aluminum, or NHRA-accepted poly box. If sealed box is used in lieu of rear firewall, box may not be used to secure battery and must be vented outside of body. Relocated battery(s) must be fastened to frame or frame structure with a minimum of two 3/8-inch-diameter bolts. OEM located batteries without complete OEM hold-down hardware must be secured to OEM battery box/tray using the same 3/8-inch diameter bolt hold-down method described in previous sentence. (“J” hooks prohibited or must have open end welded shut.) Metal battery hold-down straps mandatory. Strapping tape prohibited. A maximum of two automobile batteries, or 150 pounds combined maximum weight (unless otherwise specified in Class Requirements), is permitted. Maximums may vary according to Class Requirements.
Also a battery disconnect switch needs to be mount on the back of the car to disconnect the + side of the battery from the electrical system.
Most newer batteries are not really affected by hard launches, particularly any dry cell unit. I have always placed the posts parallel with the tail panel to reduce the possibility of the plates shifting backward and shorting, however I have NEVER heard of this happening. 
You didn't mention what rear suspension you have. With leaf springs I would run the back braces to the point in the frame the rear shackles mount to, which would be near the tail panel.
On a coil-over car I would terminate about halfway between the tail panel and the shock mount bar, with a support tube down from the brace to the shock mount tube.
Sorry your on your own on the back seat, there is no "easy" way! The good news is you can cut some of the seat supports out since no one will be sitting back there.
