I've built them a couple of ways. The 16 inch Bulgarian barrel has the gas port drilled. It is designed for a short bolt. The standard 5.45 AK-74 bolt is a long bolt. This causes the gas tube to end up short or the top cover ending up short. One fix is to use the combo front sight base / gas block from the 12.5 inch barrel. It has the gas port in the same location and uses a standard piston but the gas tube is suppose to be be slightly shorter due to the gas block design. This can close the gaps when a AK-74 bolt is used. Note the Bulgarian barrel is 23mm at the trunnion and has a wide milled receiver rear sight base. The rear sight base or trunnion will have to be modified or an AK-74 rear sight base may fit the barrel. The barrel will need to be turned down to 22mm or an AK-74 trunnion opened up if an AK-74 trunnion and bolt are used.
The Bulgarian 5.56 magazines will fit an AK-74 trunnion as will Polish 5.56 magazines. Other 5.56 magazines usually will not fit.
The 5.45 ejector or receiver will work.
The East German barrels have unique diameters and need to have the journals turned down for AK-74 parts to fit. They do not have the gas port drilled.
Galil bolts will work with AKM trunnions as will Galil magazines. The bullet guide may need some modifying to feed ammunition into the barrel.
The 7.62 AKM rear sight base is way off for 5.56 but the AK-74 5.45 rear sight base is close enough to use.
The most economical build is to buy a non-modified 223 Saiga. The Bolt and barrel will work with an AK-74 trunnion. The barrel can be fitted with AK-74 parts if desired.
Some people claim the rear of the AK-74 bullet guide needs relieving for the slightly longer 5.56 cartridge. This is not always the case.
The 5.56 case is 4.5 mm longer than the 5.45 case. Some people like to cut the ejector back towards to rear to keep the brass from getting beat up as it is ejected but this is not required.
A Romanian 5.45 trunnion takes a 23 mm barrel and works with a short bolt, such as the Galil bolt and maybe the Yugoslavian bolt (Yugoslavian bolt carrier also required).
When using a Galil bolt the 7.62 bolt carrier needs to be cut / ground narrower on the bottom to clear the 5.56 magazine opening. Some milled receiver bolt carriers and some rare Romanian 7.62 bolt carriers have a tapered, rounded bottom that may clear a 5.56 magazine without modification. Chinese bolt carriers are also the tapered, rounded bottom type.