First you have to find a '74 2.4 6-cylinder engine. Now, '74 was the first year of the 2.7 for the 911 engine, that's going to be some trick. Of course, if you mean a 2.4 engine from some non-Porsche, that's a different issue, and probably means fitting a radiator...
Seriously, do a search. Six conversions are discussed often and at length. Fitting any 911 engine from '65 to '77 will be roughly the same amount of effort.