Yes, the manifolds from the 2.25 will fit the head no problem, but there may be a clearance issue with the starter motor. The exhaust downpipe on the 2.5 manifold is slightly further forward than on the 2.25. I'm building a 2.5 engine at the moment, but haven't got as far as fitting the starter, so can't confirm if it fits or not. No doubt someone on here will know...
I think ACR do an adapter to fit an SU carb onto the 2.5 inlet manifold, but it involves some grinding out of the manifold.
What's the problem with your Weber DMTL? I was planning to use this carb on my new engine, so I'm keen to find out if it has any known issues.
EDIT: The 2.5 SU adaptor is made by HNJ Engineering.