the old chevy 4.1l/250/700r4 would make a good swap if you wanna keep an inline 6 but want the auto O/D as well. you would need some engine side mounts fabbed up on the frame to accept the side mounts on the engine, a trans mount fabbed up for the trans since the original trans cross member is under the bell housing. some sort of aux frame stiffener possibly where the old trans cross member used to be. if not the frame can flex and play havoc with the cab mounts/door to fender fitment etc. I know, there are lots of guys who simply snip the old cross member out and run it, just saying. some shift linkage or a cable shifter set up, a rad with auto trans cooler built in or a large trans cooler separate from the rad. cooler lines, and some wiring changes. if you plan to do that along with a jag front end you may wanna plan out the whole project first so you ensure the oil pan will clear along with a steering column mod or swap. the old column is part of the original steering box. some simply cut the column off below the firewall and install a bearing in the column tube there, then connect to the new steering box with some new u joints and a collapsible column "DD" rod. if doing the engine swap it would be nice to have HEI ignition and an alternator while you're at it. the HEI requires a little larger wire ga to supply ign source power. you would also need a different style of starter switch, possibly a push button, because the old step on starter pedal wouldn't work. I also advise, for hot climates, a fan shroud on the rad and a coolant recovery reservoir. you would need to do some drive line work as well to ensure the engine angle matches the diff pinion angle (oppositely). usually 3 deg down on the engine end of things. then there would be some throttle hook up work to do and, of course, the exhaust. the 700r4 needs a throttle position cable hooked up and a module or switch for the lock up torque converter etc.
here is a bit of info from a trans supplier with an idea of what you need from a trans perspective
https://www.monstertransmission.com/...l#.XjCa32hKiUk
some info on a firewall mounted pedal and master cyl/booster kit
https://www.hotrod.com/articles/1104...ooster-master/
a thread on this site showing the HEI swap on a chevy 250
http://67-72chevytrucks.com/vboard/s...d.php?t=148346
post up some pics of what you have there to work with. and a game plan if you have one.