300tdi to a LT77 ??? worth it.

Sure but that requires a complete rebuild. Might as well just pickup a used TDI LT77 or R380 from a Disco at a wreckers.

When I've looked for 300TDi's to buy for donor drivetrains I found around 5 times as many manuals as autos for sale, so they shouldn't be too hard to pick up. Having said that, to make an auto work you just need a bit of extra fuelling down low so you start the boost happening at converter stall. A tweaked TDi auto is quite a reasonable beast to drive.