Short answer? It can be done, but not easily.
The difficulty one has with 140s is that the way the seat attaches to the floor is somewhat different to the 240s and 700/900 and etc, as the standard seat mounts bolt to the body in 3 places, not 4.
Here is a (driver's side) front seat frame, with a height adjuster, from a 164. 140s don't have these as standard, but the mounting points to the car are identical.
From the top view, you can see that the front mounting point is offset left, towards the transmission tunnel - so, this is a RH side fitting. The LH side front mount is offset to the right.
This is a side-on view of the mounting frame.
On the left side of the picture, you can see a pair of brackets - one close to camera, the other near the ground - with 4 bolt holes. These brackets bolt the seat frame to a body cross-brace which runs from left to right. The seat adjuster at the front (the 140 uses a single bolt with an eye, not a mechanism like in this pic) bolts to a captive nut in the floor, attached to a reinforced plate.
The silver bits are for the fore/aft movement of the seat, but the locking mechanism has been removed from the seat mounting frame in this pic. There are 4 bolts on each rail, which should allow the fitment of an alternative seat. As yet, I haven't experimented with fitting a 240 or 740 cushion onto that frame, but there's a possibility it could be done without a lot of drama.
Fitting the replacement seats to the existing original seat frames saves a lot of drama with requiring engineering approval, which AFAIK is required here in NSW if the seat mounting points are changed.
