i voted 351 because they are a lovely engine that revs well , pulls good power even in a mild state of tune, and and pretty bloody strong , where as the rover is a good engine so dont get me wrong , requires more money to achieve the same hp as the 351, and as for the extra weight issue bigger cubic inches means usually more torque so although thew handling may be affected is shouldnt be that noticable!!
but on a serious note i would sell me the 351, then buy a dart simiese ally block, ally john kasse heads , steel stroker kit, dyers 671 supercharger,twin demon carbs and 351 kieth black piston and make your self a 373ci small block ford that with gas will produce around 900 hp and still weigh less than an essex v6!!!!! well thats what i did!!!! and it rocks!!!!!!!!!!