Simon

From those specs I would guess you are based in the UK, I'm going to respectfuly disagree with Carl, I beleive the normal alloy for this application in the UK is 6082-T6. Most reputable manufacturers use 6082-T6 for masts, booms, beams etc.

6082-T6 is superior to 6063-T6 in terms of strength, but most grades are similar in terms of stiffness.

Check if the anodising is on inside and outside of tube, if only on the outside you may want to follow Carls advice.

If you want more info on the different materials mentioned try using http://www.matweb.com

All that said, 6063-T6 is probably cheaper, so if your components are breaking in compression then you may not see any real difference in terms of strength.

You pay your money you take your choice.

All the best

Gareth
www.fourhulls.com