Talk to raintree and or WSU extension for your area on types. Your boxes need to be big enough to support the full canopy in high wind and not tip over. You also have to consider that Your root mass is calculated at approx 1 ' out from the base for every fit of height. IMHO it is better to replace your soil and drain it than go with boxes . I have 30 fruit tress all started from raintree whips 20 years ago . Some tolerate wet feet more than others. The tree usually lives but is weakened and the bugs and diseases hammer it. Pears are pesticide and fungicide intensive but there are some heirloom varieties that do well.