I agree with the proper exposure, normal development, obviously use a tripod, easy to bracket and adjust contrast during the printing phase. You mentioned that the posters are old and while they appear contrasty the paper may have yellowed so I would consider making an additional set of negatives using a yellow filter - that may seem like it will increase the contrast and it probably will but it may also give a reproduction that is the closest to the original as it was some 40 years ago and you can still adjust the contrast when printing.

http://www.jeffreyglasser.com/