These days, the spacers aren't what break usually. The most common spacer failure is the inner lug nuts loosening up.
Most spacers on the market are aluminum. Aluminum expands about twice as much due to heating as steel, and there is a lot of thermal cycling from the heat from the brakes.
So, once the spacers are tightened down, especially when they are new, then you drive, the aluminum expands, stretches the studs, then contracts, and there's some looseness in the nuts then.
Mainly, you just need to check them more often for tightness, especially the first hundred miles. I checked at 20, 50, and 100 miles, all were tight on my toyota. I use 1.5" spacers up front, to match my wider rear axle. Going to widen my front axle, though, and get rid of the spacers.
I'd think making your rear axle 5 on 5.5" would be best.