Define "mountains" (degree of slope, elevation, etc.) and tell us what engine condition yours is in. For example, if the static compression is only around 45 lbs, they ain't much on holding themselves back but they do REALLY well with a H/C head, ....except on long 14
° windy mountain grades.
In all seriousness, they do very well depending on condition and how much the engines are altered. A good rule of thumb that works very well when we drive Fords that were manufactured prior to the Model A is, "let the engine tell you how fast it wants to go up the hill, ...and go down the mountain in the same gear as what you went up the mountain in." In other words, take your time going up, --and then do the same going down the mountain and you will do just fine.