D4b minimum, offy dual 4 or even a performer. Figure out sensibly how large you can get the port window on the 273 head and compare to and make sure you can get the intake to match. The stock 273 4brl doesnt flow much without monitoring the work 'flowbench'. Other way would be a set of magnum heads...or 360 la heads with 1.88 and a good vj some porting and gasket matching then just use a rpm dual plane intake. 750 will work, around town the 650 will feel better below 5000 rpm but where the peak tq falls of with the 650 'around 4800-5000rpm' the 750 will carry on another 1000 rpm or so.