Your going to find that as the ambient temps climb the modified JL is going to want to run a cooler plug...With the timing and compression the engine runs the OD97 will start acting up in warmer weather.... Chances are a OD99 will work excelent, as its slightly cooler then the 97....Myself I have been running Picco P6TH , however my climate is still quite a bit cooler then yours...But as a general rule a medium Picco should do the trick...either a P6TH or a P7TH........There is nothing wrong with the OD97, but we are running it in a more aggressive environment then it was intended, we aren't far off the 97T's capabilities tho so a switch to the 99T may be all that is needed.........Also as a general rule a 97T will not work in most of my engines, the coils are not durable enough to provide reliable performance... In one of my Picco's the 97T's will barely last a couple of tanks before the coils start distorting and the tuning becomes erratic.....While in the exact same engine and setup a Picco P6TH will last several gallons and provide stable tuning and performance the entire time.... In a really hot climate I would run a Picco P7TH...But be aware that due to extremely thick coil wire Picco uses this glowplug needs a really strong glow ignitor to fire it...If your glow ignitor is weak or not fully charged you will not get this plug to fire..I personally run a 1.5 volt hobby battery with a cable end...These plugs are a little more expensive then other brands, but they work phenomenally well in the high compression high output modified engines...They are built with high grade heavy duty coil wires, and will be able to operate effectively under the conditions we are placing them in.......

edit.. I will add that in a cooler climate you can get away with running a hotter plug.... But keep in mind that as the temperature rises the engines need for a cooler plug will also increase...