By club racing, you mean autocross/roadrace/hillclimb, or drag racing?
I have read you guys have a more interesting idea of racing "down south"
It would depend on how hot the block got.
http://www.jbweld.co.uk/files/prodb.html
Supposed to be good to 300C
A block should NEVER get that hot.
If you have a nice clean punched hole with no risk of further cracking, isn't a structural area,etc, it should work fine.
A PROFESSIONALLY done weld would be better... but...
Consider getting some of the 101.6 autocraft jugs, rework the wasser heads, and run them water cooled. with a small radiator, like off a MC. A thin plate welded on the bottom to seal `em up is all that should be needed.... Corroded wasser heads are a dime a dozen in the US--But for this app, they would be perfect.
A pair of good electric blowers might be sufficient to keep the jugs cool with WC heads... The big bores dont have much for fins tho. Perhaps modified Duetz(sp?) AC diesel jugs?
No stud relocation required.
(Hell, almost no machining required, you'd just have to hack off the water jackets and bore the case a tad) Keep the water pump... With some creative plumbing work, you might be able to use the existing housing in place
Of course the Pauter/autocraft etc "big bore" heads will also bolt on, just cost more $$$ and wouldn't have the cooling capacity.