Probably.
Wheelbase for '68 Dodge is 122"
Wheelbase for '66 Plymouth wagon is listed as 121", but I think it was actually closer to 121.5". Seems unlikely that they would stamp out a separate floor panel for such a small difference.
Might be best to see if you could get him to measure his. The only part that is different between the various wheelbases is the rear most section, so you would only need to measure the length of the rear floor pan from the seam to the vertical rise under the rear seat.
View attachment 425223
Jeff