No, I just assumed that since the block is 32 feet long and we are building two towers under it they would need 15 foot logs. I suppose we could have a bit of overhang at each end. The transverse logs need to be as wide as the block plus enough to carry the fulcrum logs. I take the point about using modern tools. Even a brace and bit might be ridiculed. Actually, the only purpose of dowelling the longitudinal logs to the transverse ones is to prevent them from slipping inwards under the pressure of the props. If we had short timbers with both ends bird-mouthed we could put these between the longitudinal logs like noggings (for non-joiners a noggin is a short bracing timber placed between main timbers). Once the props are fitted and wedged everything will tighten up nicely. If the longitudinal logs were partly notched out where the bird-mouths fit that would prevent the props or noggings from slipping along them.