wiring for rear mount battery.. what would you do?

Most people over look the 1 thing that's most important... a really good ground. Leave the solonoid where it is & run a big welding cable for it, use a high-quality connector & solder it! Battery has to have 3/8" hold downs & really should be in a good case. Also, leave the alternator as it is & find someone(I have one but don't remember the man. name) that makes a diode to shut down the alt. when the engine is off, it's NHRA legal. Make sure to route the wires away from heat & suspension & use adale clamps to make it pretty.
70 Cyclone Spoiler 429 SCJ 4:30 Drag-pak
65 Mustang show car one-owner Un-restored
65 Mustang one-owner
66 Mustang 8-second street legal drag car
90 GT next drag car
68 Hurst Olds 455 4-speed
86 SVO
85 Porsche 944
98 Jeep Wrangler
95 F-150
76 20' E-350 motor home
Too many parts cars to mention

Although I've run the race car for years with the solenoid on the frame rail near the starter, I do see the wisdom in locating the solenoid to the trunk, especially in a street car.

Yes, if you are going to race the car, you'll have to demonstrate the emergency shut off will indeed kill the entire electrical system, which won't happen as long as the alternator is still hot (and the engine is running). I've never dealt with this matter (no alternator) but would assume you could either run the field wire to the back or install a solenoid up front for it, which would be controlled by the shut-off. The rule book likely has the specifics (I don't have a current one).

Just be sure to protect your cables from chafing and impingment by chassis parts...some guys put them in pvc or something like that (I don't)...

Hope that helps!

I would leave the solenoid in the front. Hey wait, I did, LOL. It comes in handy for bumping the motor over when adjusting the valve lash or for hooking up a timing light. Get the Summit battery kit and a Moroso on/off switch. They are both NHRA legal. Run a 6 or 8 gauge wire with your big positive. Theres a threaded hole in the frame rail under the right rear side that is perfect for the short ground cable. The big red cable goes up over the fender well and down inside the passenger side rocker panel and up behind the kick panel and out which ever hole up under the dash you prefer to use. You can drill holes in the fender well where the inner and outer are put together for tie wrapping the cable. It isn't going any where like that. Well, this is how I did mine but you don't have to do yours that way. I had to grind my spare tire hook bracket thing off for the tray to sit level.

Oh. Just put your alternator charge up wire on the battery side of your kill switch.