Transmission help

Im am in the throws of upgrading my 1976 f100 from a 302 cleveland with the "funny ' four speed , ( similar to a top loader with a really tall first ), to a fairly warm 351c with iether the c4 or a c6 , cam chioce I shouldnt need a hi-stall however I will be using a trans cooler (A) for a bit more oil volume and (b) obvious reasons.So guys what do you suggest the prefered transmission should be and why .Both boxes are readily avaliable for around $700 aussie . which is longer ? and will I need to change tail shafts ?( one piece ), thanks. I should also mention this truck is my work truck every day use and tows a tool trailer running on LPG.