how much trouble if possible would it be to install DCC decoders on these DC locomotives and if possible what are the decoder numbers
1. A bit more information would be helpful
2. If you are asking that question then it probably is not wise to try installing DCC
If the locomotives have soldering pads then you will need the ability to soldier multiple wires to the board and keep track of what each wire corresponds to with the locomotive.
If there are none then you will also need to isolate the motor as well, and things become even more difficult.
For some N scale locomotives it may be advisable to use a Z scale decoder instead of an N scale decoder depending on space constraints.
Without more specific information; however, I can't give you anymore assistance.
I assume you mean EMD SD70? What are the locomotive brands?
-Mark