dude man do what you want but its more trouble than its worth yeah it will run fine first but as it frees up that sludge its going to get clogged in your oil pump filter and can cause your stuff to blow

if theres no sign the block is cracked your in good shape and it sounds like you also need a temp gauge in your hose going to your block from the raiatior

yeah man a ford 351 windsor will surly fit i own an auto motive shop and i put a 351 windsor h/o motor in mine instead of that little 4.9 but if you go to the clevland be prepared to pay out the a$$ cause there a sought after engine hard to find and any way that 351 windsor HIGH OUTPUT is turnin more rpms its the way to go