There's probably lots of reasons why Alfa's aren't that popular in Rallying, but I don't think the Alloy sump is one of them to be fair.
Anyway, good luck with the 147 I think it will be fine as a road rally car, I do a lot of Road events around the North of England and the only Alfa I can think apart from Historics (with Alloy sumps of course!) is a 145 QV which is run by Jem Dale of Clitheroe and District MC, it's very quick too, won our class on the Clitheronian last weekend. We've had a few head to heads with him and he was as quick as our Clio 172 on the faster sections.
The main thing you need to look out for on the 147 I reckon will be damaging the suspension on the rough Whites, it's a bit delicate that stuff.
Should be plenty of clearance under the arches for some 15" knobblies.