A 1960 235 will fit in a 1951 pickup. Someone else will have to reply about the front motor mount (all of mine in 1/2 ton trucks have two bolts in the front motor mount). The '60 engine's front plate can be easily adapter/drilled in order to get it to work.

You will need to use your '51's clutch housing and, if you will run at 6v, your will need the '50 flywheel and clutch assembly.

Also, you will have to deal with the longer (to the front) water pump on the '60 engine. The options for the water pump are:
- buy a short-shaft water pump with the correct pulley groove($$$), or
- get a pulley with the right groove and push in onto the '60 water pump and cut off some material off the pulley post, or
- buy a relocation plate and drill a few holes in the block (this allows you to use your '51 water pump).