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You will need a TF727 with the circular New Process bolt pattern for the Dana300 to bolt to. I think the only factory 727 that you will find that bolts to a D20 is a pre-79 International Harvester version, like the one in Scout IIs. To use that setup, you would need the output shaft and adapter from the IH version installed in a Mopar version that has your engine bolt pattern, and the IH input gear installed in your Tcase.....I think.
Can't you just buy a transmission/Tcase combo from a Dodge truck and use them both? You won't have a D300 or a D20, but almost any fullsize Tcase will hold up fine in a Jeep.
As far as strength goes between the two cases, I think the D300 is supposed to be stronger, but I have seen more snapped outputs on D300s. I guess the lower low-range puts more stress on it.
There is also a clearance issue with the TF727 in a SWB Jeep. The pan is really wide and interferes with the front driveshaft. I think at least a 4" susp lift is needed to clear the pan. I haven't done the swap, but it seems like I have heard or read that somewhere.