I think the sand blasting would be a good route, then used red oxide like yella says or that cold galv spray its brilliant.
I would still recommend rubbing down the seams afterwards and seam sealing it with proper car body shop seam sealer. £6 for a litre can and I am still using it on random jobs. You can use some white spirit on your finger when its fresh on to give it a smooth look - however the only ones that are visible are the footwells but by the time you put a mat in there you won't see it.
Obviously being a body shop sealer it over paints perfectly - So you can give the whole thing a coat of a good primer and then lay on the top coat, engine bay side of the bulkhead I gave it about 4 coats, footwells got about 10 coats, door pillars I don't know I just kept putting more on after it dried. You can also stuff the channel sections (door pillar, under & above the vent flaps) with waxoyle.
Remember to order door hinge captive nuts, does your door pillar have the captive nuts in it for the wing? If not get these, also get new wing bolts (very course threaded things which will probably not come out of the old BH properly) I ended up running a 9" angle grinder down the length of the old bulkhead and then just battered the bolts from the other side to ease them up and they soon came out - don't even waste time trying (make sure the courtesy light wiring is out of the pillar first - it comes in from a hole on the top).
I would also get a new set of door hinges, unless your current ones are in good order. I don't see the point in doing all this work then putting old rusty hardware back on. When you get the hinges like that you can paint them up during your bulkhead swap so come fitting time they are fully painted all sides. Remember hinge gaskets.
Where are you planning on doing the swap?
There are a few ways you can do it which will depend on help and space available.
You can strip the whole lot down in situ so by the end your lifting a naked bulkhead off the chassis - not the best if you have to do the swap outside.
Or
You can strip the the landrover enough to get the bulkhead out, remove all the connections, then get some strong people to help you lift the whole thing off and sit it somewhere with good light and if your lucky enough heat!
You can build a basic frame with timber to hold it for you while you do the hardware changeover you can sit the two next to each other and transfer the lot over.
Take your time and don't rush it, it will be worth it in the end, heater box out... sand it, give it a paint - I had a couple of cans of Satin Black spray paint and just blasted everything as I went... door catches, heater box, expansion tank, radiator mounts, brake servo...
I WILL post the list soon - its still in paper form somewhere.
