Welcome. I moved your thread to a more appropriate section. Have you done a thorough assessment of the 345 condition? They're pretty durable engines and have been known to faithfully log significant miles. What other areas need attention on this rig? Usually the list is quite extensive. How do you plan to primarily use it? Would a good tuneup buy you some time to focus on other areas of concern?
The IH sv8 engines are quite costly to rebuild in a thorough and proper manner. It is difficult and costly to realize dramatic performance gains above stock from these engines. They breathe poorly and the rotating mass is quite heavy. It takes a fat wallet to get a 345 appreciably above 200 hp at the rear wheels.
Do you have a fresh, 4 bolt main 350 long block ready to drop in? Everybody and there aunt martha has swapped a sbc into their Scout. You can find build threads everywhere for guidance on that process. What about a transmission? Whatever you have now isn't going to simply bolt up to some other block without being adapted or replaced. If you swap the trans also, then you have married transfer case mating issues to overcome. None of these hurdles are insurmountable. They've been dealt with numerous times in a variety of ways by Scout owners going back more than 20 years. The information is out there.