Yes the front frame is substantially different. Crumple zones were added so the frame is longer. The frame is about the same width overall but the newer frame rails are thicker, taller, and wider. The gas tank locations are altogether different so the fuel filler locations are different as are the body shapes to accommodate them. The newer vans mount their spare tires underneath so the frames include clearance for that. Front axles are further forward (wheel wells protrude into floorboards less) but wheelbases are very similar so the rear axle is also moved forward. Engines are moved forward so interference is likely with doghouses, radiators, etc.
I'm not saying it can't be done but anybody attempting it should be prepared for a fully-involved re-framing with entirely custom mounts for most everything in contact with the frame. Driveshaft(s) will also probably need to be reworked since either axles or fenderwells will need to be relocated and the bodywork is far more expensive/time-consuming. Engine height and length will probably require some custom work on the interior as well, hopefully only in areas hidden by the doghouse. Transmissions would likely differ, as would column shifters and linkage.
If you go ahead please keep us posted. This would be very interesting to watch.
