Richard....
1st up......e-mail your supplier & confirm that no Billings booklet was included & requesting a replacement.....[in worst case it may be available on line]
Is there any mention of ballast to be added to the hull?.....if there is, you could consider using this alternate weight to reinforce the inner surfaces of the hull with vertical planking using thin [say 1mm thick] planks with some horizontal strips following the line of the top of the hull
You will certainly find many examples of such wooden fit-out on WEB pages and this will provide a surprising increase in hull rigidity
No having said all of this, ballast is usually employed low in the hull for stability......so you must understand the type of hull and consider the equivalent weight of wood say 1/2 above the waterline
One simple test is to place the open hull in the bath, then gradually add unopened food tins [or 375 ml beer cans
] then continue until your desired waterline is achieved......add up all of the weights & there you have the total allowable mass you have for the
complete fit-out
Derek