How can you tell what size a trans is? is there a stamp of a vin or what else can I do while looking at it to determine what model IE 350, th400 ect... It is not the one in my car it is one that was for sale from a '79 but the engine was not origional either so the owner was not sure if the trans was original williammoore22008-09-12 19:04:35

The quickest way to tell a 350 from a 400 is the location of the modulator...on the 400, it is at the front, pass side, by the filler tube, pointing towards the outside of the car. On the 350, it is at the rear, pass side, pointing to the rear of the car.
Another quick check is the governor cover attachment method. The 400 gov. cover has 4 bolts holding it on, and the 350 has just a round "cup".
hth

The easiest way for me is the shape of the transmission pan. Here is a picture side by side of the TH400 and the TH350:    http://www.off-road.com/trucks4x4/article/articleDetail.jsp?id=200796