Clearly a bike.
Some good reading on heat prep:
Simple strategies to hammer strong in rising temperatures
www.hammernutrition.com
Learn how to solve common dehydration issues that many athletes experience during training and racing.
www.hammernutrition.com
IMHO, ambient condition acclimation should have already started before this topic was read here in this thread.
No joke, we all have experienced it. Even just prerunning. On the bike, no water since the bottom of the summit (and outside temps were now three digit), at the top of the lakebed my water tender ripped off a front arm and was now no good to me. Ripped to Borrego and spent some time passed out under a race car trailer, trying to figure chit out. Some chasers gave me a soda (no one had water!) and fuel, so off I went toward VDT, where around the bottom of San Matias wash I found help coming south to tend to the truck. Lots of wrong unfolded as the day progressed.
Even with the best plans with well thought contingencies, you never know what can happen. In Baja, "it" can unfold in ways you can't dream of.
Momma nature is too powerful. Make a plan, think it through, prepare it deep, and execute only what you need to win. Stay super safe and best of luck to all B500 competitors!