"I’ve seen similar setups in engine swaps with the radiator fill cap inline like you have it. I’ve also seen inline t-stat housings with and without a fill cap function fine where you have your fill cap. Those that use the inline t-stat/fill cap simply do away with the oem t-stat housing and have the lower radiator hose direct to the oil filter adapter housing with a small branch off it to the coolant reservoir. Then they either block off the bypass line at the coolant crossover or do away with the coolant crossover and use AN lines to a Y fitting at the inline t-stat housing.
Regardless, the same holds true as I described with using the bypass type t-stat housing. When the t-stat opens to allow flow from the radiator, it closes the bypass hose inlet from the crossover pipe. That’s why the t-stat has that large disc on the end of it. When the t-stat is closed (cold) - the disc is away from the bypass opening and coolant flows through the t-stat housing from the bypass hose from the coolant crossover to the engine through the oil filter adapter housing. When the t-stat opens (hot) - the disc moves to choke off the bypass opening blocking flow from the coolant crossover which of course now means coolant flows through the radiator then through the t-stat housing to the engine.
Think of it as a temperature operated 3-way valve. If the bypass is not closed when the t-stat is fully open you have a short circuit of coolant flow from the crossover pipe through the t-stat housing along with the flow from the radiator. This takes away from the radiator flow and hence the higher operating temps.
From what I see when you get the proper t-stat you should be fine. "
