I’m afraid the only work I’ve read focused solely on the Huns is Otto Maenchen-Helfen’s old tome. As one piece of evidence he notes the conflations of the Bulgars among contemporary sources. So, for example:
Sidonius’ letter confirms our thesis about the prolonged stay of Huns in the Bakans, but neither Casiodorus nor Jordanes could refer to the 470’s as “the present day.” The phrase makes sense if the Huns were the Bulgars of 505, when Ptzia and his Goths defeated Sabinian’s army, which consisted of ten thousand Bulgarian horsemen. Cassiodorus, writing his Gothic history in the 520’s or early 530’s and Ennodius d. 521 ) repeatedly calls the Bulgarians “Huns.”
Of course that could identifier by geography, a common habit of classical sources.
But beyond that there is the tradition, noted by Captain Amazing above that Attila’s 2nd and 3rd sons are variously listed as leaders of tribal sections identified as the source of various Bulgar groupings. Quoting wikipedia here:
*According to tradition, after Ellac’s loss and death, his brothers ruled over two separate, but closely related hordes on the steppes north of the Black Sea. Dengizich is believed to have been king (khan) of the Kutrigur Bulgars, and Ernakh king (khan) of the Utigur Bulgars, whilst Procopius claimed that Kutrigurs and Utigurs were named after, and led by two of the sons of Ernakh. Such distinctions are uncertain and the situation is not likely to have been so clear cut… Indeed, subsequently, new confederations appear such as Kutrigur, Utigur, Onogur / (Onoghur), Sarigur, etc., which were collectively called “Huns”,“Bulgarian Huns”, or “Bulgars”. *
Then there is the conflation of the old Hunnic language with that of the Chuvash/Volga Bulgars as Johanna cited above and the connections between them and the western Bulgars.
So it is all somewhat speculative and it seems based on a sizeable collection of circumstantial evidence. But ethnogensis questions tend to be tricky anyway. Were the Bulgars or the separate tribes that later became the various Bulgar groupings part of the original Hun group that entered the western steppes? Or were they swept up into confederacy after the fact, much like the Mongol Golden Horde was largely formed through the absorption of Kipchak Turks? Dunno.
But any rate the connection has apparently been mooted about for some time. Maenchen-Helffen notes that C. Schirren spotted what he says is Jordanes’ conflation of the Bulgars and Huns in Jordanes’ Getica, back in 1856. So, when Jordanes says…
Beyond them extend above the Pontic sea the territories of the Bulgars, whom the punishments of our sins have made notorious. After these the Huns, like a cluster of mighty races, have spawned twofold frenzied people.
…Schirren and Maenchen-Helfen say that here Jordanes is using Bulgar and Hun as synonyms.