If it where me, I would ask him!
Epoxy is very good, and the only one on that list that can structually gap-fill, it also doesnt not require a clamping force for a good bond and is fully waterproof, unlike PVA. But I would say the final bond very much depends how clean the old joists are, and its also a sod as you have to mix the two parts, well, in fairly small batches.
Ive not done it, but I would say the best option in terms of balancing easy/effectivness is polyurethane (PU) , bascally the same stuff sprayfoam is made from, only just in a tube/bottle which you apply, its then foams lightly on the job, bonding where there is contact, and foaming to fill the void semi-structually where the is not. I dont think any one brand is any better than another really. But your after PU wood adheasive, not the sikaflex/tigerseal type adheasive/sealent in guns
I dont know how feasable it is, access, and presents of nails etc, but if its filthy there may be millage in running a power plane over the tops of the old joists to get something to get a good bond to. Your aiming to stop it sliding, at all, so you get the stiffness of a 6*2 instead of a 4*2 and a 2*2 ontop of each.
- 6*6 = 32
- 4*4 + 2*2 = 20
Daniel