A post about "Andy Jennings" dashes on the MGE board got me thinking... have a look at the first two pics here: http://www.mgcars.org.uk/andyjennings/. Now, consider if you have or buy a metal dash and your old cracked padded dash. Seems like it'd be really easy to strip the padding off the old/cracked padded dash & cut the part of the metal backing that mates to the cowl off. Then use that piece and a filler piece to fill the gap to the metal dash. You'd then have a stock looking metal dash that'd bolt to the existing padded dash cowl - and it'd be way easier to do that than to cut and weld the cowl (especially without warping it near the defrost vents).
Too late for me since mine's done, but thought the idea might help someone.
Rob,
You'll probably have to open up the original dash a bit to fit gauges, wiring and glovebox. Sounds like a good plan though.