I would do the big block swap in a heart-beat since I have a 74 D100 factory 440 and a 77 4x4 Ramcharger with a factory 440, i'm also stuffing a 440 in a 1980 Mirada right now with about 6 more 440's laying around) and absolutely love the low rpm torque! On the exhaust I would steer away from using the factory pickup exhaust manifolds since they are very restrictive and they also tend to always burn out the middle of the valve cover gaskets and burn the paint on the valve covers since the manifolds tuck up so close to the valve covers. On both of my 440's I use the 78070 Headman shorty headers and they clear everything very well the only issue is you need to run the smaller nippendenso starter to clear the headers.