Rob, I did a similar job for a customer last year. I used a ST rad.
Remember though, the SBK rads and ST's are curved. This will cause big problems with mounting and clearances etc if yours has the flat rad fitted.
It's not a job I'd carry out on a "mint" bike, IMO, as it doesn't look that pretty and would detract from the value, authenticity etc . The rad sizes are also smaller, slightly. This was OK on the bike I did as it was track day only but road riding could cause problems in town etc.
If your bike has a flat rad, then I have a brand new one for a lot less than £800