ok.....ive installed my battery in the trunk of my car.  now when its hot....it wont crank. i have it grounded back to the motor with 2/0 wire. is the wire not large enough? heat soak on the starter?what do you guys think?  

I've installed batteries in the trunk of all my cars. The first one had this problem. First don't just move the stock battery back there, bump up the cranking amps to compensate for the losses of the longer wire. I've been running the Optima gel cells and they have never failed me. Second, I ended up running the main negative cable straight to the block, but it sounds like you already did that. Also just run a good grounding braid from the block to the chassis. A 5.0 should be an easy engine to start unless you've really bumped the compression. Aftermarket headers running a tube close to the starter may require a heat shielding blanket for the starter. That's all I can think of.  

One more thing... Use 1/0 gauge cable. And if you want the best, go to a welding shop and pick up some OFC welding cable.  

I agree with 67ta about the grounding and bigger battery. I have 2 980CA batteries hooked up in parallel (thats 1960 cranking amps) to crank my engine over in the winter and to keep the stereo happy. A welding shop is the best place to get wire too but you defiantly want to stick with the 2/0 cable because it is larger than 1/0 cable.  now if you are running #2a.w.g.(American Wire Gauge) versus #1AWG, #1 is larger, but then the sizes flop and 1/0 is larger than #1awg and 2/0 is larger than 2/0.  At a electrical suply house or welding shop 2/0 is called 2 aught. At a stereo shop they call it double zero gauge, they are the same guys who use NAUS instead of N.O.S. or nitrous
I just don't want to see someone accidentally buy $200+ of wire to find out its the wrong size!