1970 "heavy Half" trans change question

[art.torralba]

I've got a 1970 with an inline six and three speed column shift. The rear mounts are on the bell housing. How can I put in a '88 700R4? I have the donor trans, the trans crossmember, flex plate & starter. Do I need to eliminate that bell housing mount and just put the rear crossmember at the rear of the tranny? Will I need to shorten the driveshaft and change the yoke? Or I could just rebuild the column (it sometimes jams between shifts) and keep the three speed, maybe put in a floor shifter (I put in a new clutch).
I plan on keeping the six, but I'm working on fixing all the fuel/vacumn lines, it's got a bad vacumn leak somewhere. The previous owner changed the distributer to an HEI unit, what should the plugs be gapped at?
So many questions, not enough time (and money).