Will an LA-360 bolt in where a LA-340 was?

69 will have biscuit style mounts. I think all 360 had spool style (truck might have used biscuit, not sure). You can use 318 biscuit mounts but will have to add washers to one side to take up the difference in the mounting ears on the block. This works fine, done it many times.
I would get a B&M solid flexplate for 360 balance. Other than that it should be a bolt in deal.