Rule 40B requires not just a win in a state but that you get 50% of the delegates for that state. What four states remaining do you think Cruz will win a majority of the delegates from ? Especially with Kasick still in the race. There are only MT & SD left in the winner-take-all type that Cruz could win and without WTA he is unlikely to get a majority of delegates.
I do think Trump will get over 1237, but there are definitely ways he could fail to get enough delegates even if he wins every remaining contest. With 3 candidates, it could go 40/35/25 in every state, Trump wins them all but has to split delegates except in the remaining 5 WTA contests, and Cruz never gets another majority of delegates anywhere.
Cruz is likely (nearly certain) to get a majority of delegates from Utah, Indiana, Nebraska, South Dakota and Montana.
In addition, even though their delegates are unbound, it's quite probable that a majority of delegates from the North Dakota and Wyoming would choose Cruz, and I've never heard anything about Rule 40B that it applies only to state delegations that are bound.
However, it wouldn't take a lot for him to miss... if I'm off just a hair in one WTA state, or in a dozen of WTA congressional districts, the flip to Cruz could be more than 32 delegates.