You can check it one of two ways......(motor and trans warmed up of course):

-Engage the transbrake, floor it with no chip in the two step(if you run one).
Hold it long enough to get a good reading(like 2-3)seconds.

Or

-with the car rolling along at about 10-15mph in high gear, floor it and observe where the tac levels off.

You really want it to be at least 4500rpm for that application.

IMO, this needs to be a known quantity before any other changes are made.