It looks like the way you are supposed to connect the primary is, if you want the 3k load, then only use the two taps that are labeled 3k and ignore the rest. Same goes if you only want the 2.5k. Just connect one of the taps to your plate and the other to your B+. Even the website you had referenced earlier on seemed unclear as to what the center tap would be used for. If you want a 5k primary then use the 2.5k taps, and then on the secondary, connect your 8 ohm speaker to the 4 ohm tap. This will give you 5k at the primary.