Like Scarebird says, they may not dim at all or dim the way you want.
You can certainly use LEDs but get specifics from someone that used them and is happy. It may be tricky to get the right "light output power" and the "color" can vary.
You may need to re-do the blue reflector paint in some of the gauge backs.
Not a 70 Tempest, but this may offer some insight:
https://forums.maxperformanceinc.com...highlight=1891
It has been almost 10 years since I went through my dash, and I chose to stick with incandescent lamps.
Here are some notes I took, but they are incomplete. No guarantees about accuracy so double check:
There are several threads on here that suggest alternative lamps that are brighter. The tradeoff will likely be hotter and less life. Here is another chart I pulled out of my notes that details some of the differences. For example, 168 is sometimes recommended as a brighter option for 194 and is what I installed:
