Good advice, the 221 with 2v head ive done, and works well, but the 3 speed, not really into slushies, Id go with 5 speed, and if you cant get to that money wize, a single rail will be cheaper and easier as bellhousing etc are easy to find. XD XE? cable clutch v8 boxes are better as they dont have the lower first gear that six cylinder ones do, Id try to keep above 3;1 in first gear, a top loader is great and was used behind 221s in XW.
The comet axle is a good idea, as long as you can get the ratios etc you desire, 3.23:1 is a good start with 14 inch wheels and the six.
A7M
PS just notice "clint" in a post below has just advertised a heap of manual bellhousings etc for the sixes, I dont know where you are, but Im in Perth and may be somewhere near "clint" if that helps