Decide what gearbox to use. I opted for sprint as substantially stronger. So I went with a sprint axle and prop too. Not sure how swappable all those parts are (ie which if any prop would fit an 1850 box with sprint axle, or the other way round)
Once you have decided which major parts, you will need to either swap the front subframe, or I did the alternative (often frowned upon) and chopped the OHV engine mount towers off, plated over and bolted OEM jag e-type engine mounts to the subframe. Then fabricated brackets from the engine to the new engine mount (couple of hours to do all that, very simple if you weld)
I also went alternative with radiator, and used a passat rad (£28 delivered, very pleased with it) and a bit of replumbing including a header tank. The obvious answer is all dolomite stuff, but it is getting expensive....
I also had to change the gearbox tunnel, should be available or it is possible a OHV type could be modified.
I am sure other will have their take on the conversion too

Gearbox choice is really down to how you drive. Mine gets a hard time, and I REALLY want to get out and do some Targa rallies soon. When I get enough free time at a weekend to coincide with my Navigator...