To My knowledge, the only differences between a series 2a and 3 chassis are the gearbox crossmembers (series 3 clutch slave cylinder housing fouls on 2a chassis member), check strap mountings over rear chassis and the front (thus applicable to you) brake pipe arrangement. Not quite sure how its different, sure a search on google or such would throw a couple of useful hits. But i'm sure that its nothing that can't be sorted out with a bit of thinking..
Matt