As I ponder the aspect of the erection of the church frame that has consumed my thoughts now for a long time, and DL your thoughts about dovetail attachment-- well taken-- I am putting forward another scenario for thought, I hope that others might comment that have some thoughts about this subject--
As I look at the ceiling network of timbers, it reminds me somewhat of the 3rd floor of the grist mill where the cross girts supporting that floor, mortises into the top plate from the side, the whole timber framing setting on top of 36" stone walls, this style of framing making use of mortise and tenant fastening.
If I was given the job of erecting this church some thought would have been directed to fabricating a raising bee that would go something like this--
The two long centre ceiling timbers, creating a unit-- married to their short centre connecting girts, this unit elevated to the height of the upper wall plates, sitting on 3 temporary scaffolds,
Next the end bents would be raised, sliding the tenants of the long ceiling timbers into their mortises on the end wall upper plates
Then the side walls would be raised, and the remaining ceiling girts would be slid into their waiting mortises
Lastly all the wood pins would be placed
The temporary tressles would remain to be used to support the ceiling structure until the rafters are placed, and eventual supports for the ceiling put in place, whether it would be rods from the rafter peaks, or other types of supports--from the photo it appears that trusses were not used