I'm rebuilding an old road bike we found in a skip.

It was decent quality with Shimano 105 or 600 bits.

Question is:

The 7 speed cassette has teeny weeny cogs and seeing as the bike will be used in a mountainous sportif I want to fit a cassette with a lower low gear. Like an 11-27 or something.

Is there a way to determine what range the Shimano 600 rear derailleur will handle?
I realise that I could stick on an MTB long cage derailleur but it would spoil the look. I don't want to junk any parts coz once I'd cleaned off all the grime everything is actually in really good nick and it would be nice to keep it original so it can be lined up next to loads of Carbon Cervelos on the day having cost nothing more than some bar tape, cables, chain and tyres.

Try here?

Most Shimano rear road mechs will cope with a 28T but you do need to be careful how much chain you need but if you're using a double up front rather than a triple it should be fine

Cheers for the answer.

Yeah it's just a double up front and I'm fitting a new chain so should be cool.

That Shimano link was great although doesn't cover older stuff.

I guess you can change cage length by swapping a longer cage for a shorter one too.

Perhaps compare cage length to a more recent one that you can find the specs for ?