Will the Bulgarian kits work with this receiver,
or are the barrel trunions different than the receiver?

for the euro kits, the trunnions must be narrowed. this is easily done with a milling machine. the rivet pattern will also be off and require tig welding the mis-matched holes in the receiver closed, frinish grinding the weld and drilling new holes to match the euro trunnions.

What kind of guns were these built into when they were sold?? Chineese??

yes. clayco, kengs and several importers were bringing in chi-com ak parts sets. some of those same kits are still for sale (sales were very poor on the kits, as very few/no parts were available back then to complete builds on. clayco wholesaled out kits and b-west brought in some for their builds.

What about the Polish kits i have seen lately ?

imo, getting a chinese parts set is the easy way to go.