Guess you're not a purist, then? 😃
In which case I'd be more tempted to swap in a factory disc brake rear end from a later model 1800, and swap the front wheels / hubs / brakes over the later 5x108 wheel spec, and the later mode Girling 4-piston front calipers, etc.
The advantage of swapping over to the 1970+ disc brake rear axle is that you get a Dana 30. AFAIK, the swap is not a huge job on the 1800s. Using the Nordicar disc brake conversion kit as arebee suggested means you're stuck with the old Dana 27 diff, and there are a lot more choices of ratios and LSDs around for the the stronger Dana 30.
Process of conversion is broadly described here, on Phil Singher's pages -
http://www.vclassics.com/archive/qrdisk2.htm
My 1800E is a 72 model and already has the disc brake rear axle. I won't be trying to shove so much power through it to justify a 8.8 Explorer diff or similar.
Suspension-wise, I'm still to make many decisions; for the front end, I'm entertaining thoughts of a custom-made double wishbone setup with a rack & pinion steering setup (to hell with purity - I'll keep this car until I die). My car is it's currently LHD, and LHD cars aren't pleasant to drive in LHT countries like OZ. RHD steering boxes for 1800s in good nick are old, rare, and hideously expensive, and I'd likely spend as much to get one as I would on a custom-built crossmember with double wishbones and RHD rack & pinion anyway.
For the rear.... well, if I can find a suitable IRS at the correct price and correct size, I might get things done that way. If not, the original 1800E rear suspension with the Bilsteins I bought a while ago, some more sensibly-rated springs, and fitting a rear sway bar that acts like a torsion bar between the two trailing arms (like the 240, and not the iPD type of rear bar that mounts to the floor of the body), along with some fresh bushes, might work out just fine.
