FWIW, I drive my '49 TC all over the west coast, sometimes 2-3K miles at a time, and sometimes with stock TDs and TFs following along. The TC has a stock engine and gearbox with slightly higher rear end gearing. The engine does just fine. If the TF is a 1500 then you will have more power than my car, so perhaps a gearing adjustment would be enough. Installing an MGB gearbox would certainly detract from the value of the car, if that matters to you.
As much as I prefer period-correct parts, a 5-speed would likely be a much easier conversion if you want more than four gears.