Though it will be difficult to prove and may be a lengthy lawsuit, I believe Smithson would win for the following reasons:
A verbal agreement can lead to formation of a partnership contract
Both the parties, in this case, Smithson and defendants had full idea about the agreement
As with a normal contract, there was an offer and acceptance. Frank White’s firm would finance the purchase (offer) and Smithson accepted on the condition he would see engineering of the property (acceptance)
Further, the agreement was made in good faith. Smithson had no idea that the defendants would go behind his back and make profit for themselves alone
The following proofs Smithson can use to prove his case
He can prove in court that he had contacted Monsanto Chemical company initially
He can show proofs of his contacts and follow-ups done with Frank White
He can present the sketch maps he had prepared with the proposed layout of the lots, roads, and so forth as proof of his groundwork