I think you have two choices.
The easiest would be to find another motor. I'm not sure what the cost or what might be available in your area though that might be a factor. If you can't get a warranty on it. Then I would want to hear it run and know what the oil pressure is. If you do go this route you can then take your time rebuilding the 283.
The other option would be to change the rear axle. There is a problem with changing the rear axle and that is the lower gears are going to put additional load on the engine. It looks like you have an SM465 which does have a granny gear. I swapped out the 4.56 in my 68 for a 3.73. Second went from about 15 mph to about 25 mph at my shift point. Perhaps some else has an idea.
LG
