Heat. That is my concern also... additional heat as well as the additional draw on the cars 48 year old wiring. But I ran these new 6W bulbs in the tachometer since April with no ill effects. (I didn't finish installing the remaining bulbs until just this weekend.) Below is a picture from April showing the new bulbs in the tach while the old bulbs remained in the rest of the instruments....
I purchased the bulbs from Candlepower.com here:
http://store.candlepower.com/38xefiinpala.html You will need 5 bulbs total to replace the existing bulbs in the 3 main gauges. I purchased extras in case I had any D.O.A bulbs from the retailer.
Note: As with even the standard bulbs, the speedometer is slightly less well lit then the tachometer. That is by design. It's just the way the bulbs are situated behind the gauge face that causes this uneven lighting. Not much can be done about it.
As to the question about the center gauge having two bulbs, mine only had one which is located direct center in the rectangular gauge cluster. Because the bulb is located in between the 4 small gauges I decided the best way to access it was the pull out the oil pressure gauge (which is held on with two small screws on the back side of the gauge) so I could gain access to the bulb. It is a tight fit trying to remove that bulb with bare fingers but it worked fine and the new bulb slotted in with ease. The center gauge does have other bulbs (turn signal, low fuel light etc...) but they are not related to the main objective of overall gauge lighting. Once installed, I recommend carefully wiping down the bulb glass if you have to handle them with bare fingers. Right or wrong, I have always heard that the oils from ones skin can cause a shorter life for the bulbs.
There are numerous excellent write-ups on this forum about how to remove the gauges so do some searching. Most all of them comment on how difficult it is to remove the gauges from the dash. I did not find this to be true at all. Sure, it is a tight fit reaching up behind the dash to first undo the tachometer but from there, it really isn't a big deal. Whole project was done in less then 2 hours and that was including removing the chrome bezels so I could clean the glass and inside of gauges.
Next: Remove the HIVAC (Heating Ventilation Controls) controls and install new bulbs in there. My controls work fine but the current, original 2w bulbs barely qualify as lighting and the Lucite controls themselves are so filthy they are not illuminating as they should. Will try and post pics of that project when I can.