support for additional 70al CW tank

Could I please get some advice about installing an additional 70gal CW tank. (330kg inc tank I think)

First picture is position of current 50 gal tank, but refill/draw rate is not sufficient, with two teenagers and power shower X2 in the house. Seems to be supported by 2x4 sawn timber (OC unknown) and 18mm chipboard across the purlins, plus one 3x3 original beam.

2nd picture shows vaulted ceiling space I want to use (Pic1 bathroom through door on left). House is C1900 all internal walls are single brick full height of house, so assume load bearing.

Wall to Purlin is 1.8m and fitting 1650x610x500mm l.w.h tank leaves just enough room to use 2x4 CLS and 18mm ply directly above the door frame height. Had a look at span tables and its on the limit for floor joists. (http://www.awc.org/calculators/span...bmit=Calculate+Maximum+Horizontal+Span#answer)

Platform will need to be 800mm wide to allow for plumbing.

1. Will 2x4 CLS and 18mm ply be strong enough with o.c. 300mm and two rows of noggins.
2. Plan to fit joists to purlin with long leg hangers, is this feasible.
3. What is best way to attach joists to wall end (masonry hangers or through fix a timber end plate)
4. Should I through fix a joist to side/long wall to spread the load on to that wall.

any advice welcome.