Do some research, look at some stock trucks, get a shop manual and perhaps an assembly manual before you decide to put it back to stock. In many cases the rear cross member has been removed for an automatic trans and as mentioned the clutch parts are missing. There is a good chance the stock front mounts have been changed to side mounts. In this instance the engine will not even be in the stock location. You will have additional issues with a driveshaft as well. There are still a lot of stock parts out there but I would recommend rounding them all up before you tear a running vehicle all apart.
Fred
It appears that this post has been duplicated in "driveline" so much of this has been covered already.

Last edited by rfs56trk; 02/10/2014 6:25 PM.

1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine