Hello,
Putting a small block in your '61 will require a little bit of work as the v8 bell housing is different and the motor mounts are also. I would look for a 63-66 C60 in a boneyard and rob the necessary parts although they are a little diff. than your 61 they will have the motor frame mounts and bell housing you need. Swapping out the old 235 is probably a good idea. If you do much hauling as it is way underpowered for a truck this size. Mike