Because of the extensive modification I expect that C&R rules do not apply. I'm not a lawyer, but here is what I think is applicable to the situation as far as classifying the rifle.
From ATF Ruling 85-10
In classifying firearms as curios or relics under this regulation, ATF has recognized only assembled firearms as curios or relics. Moreover, ATF’s classification of surplus military firearms as curios or relics has extended only to those firearms in their original military configuration. Frames or receivers of curios or relics and surplus military firearms not in their original military configuration were not generally recognized as curios or relics by
ATF since they were not of special interest or value as collector’s items. More
specifically, they did not meet the definition of curio or relic in section 178.11 as firearms of special interest to collectors by reason of
a quality other than is ordinarily associated with sporting firearms or offensive or defensive weapons. Furthermore, they did not
ordinarily have monetary value as novel, rare , or bizarre firearms; nor were they generally considered curios or relics because of their
association with some historical figure, period or event.
https://www.atf.gov/files/regulations-rulings/rulings/atf-rulings/atf-ruling-85-10.pdf
Curios or relics.
Firearms which are of special interest to collectors by reason
of some quality other than is associated with firearms intended for sporting use or as offensive or defensive weapons. To be recognized as curios or
relics, firearms must fall within one of the following categories:
(a) Firearms which were manufactured at least 50 years prior to the current date, but not including replicas thereof;
(b) Firearms which are certified by the curator of a municipal, State, or
Federal museum which exhibits firearms to be curios or relics of museum
interest; and
(c) Any other firearms which derive a substantial part of their monetary
value from the fact that they are novel, rare, bizarre, or because of their
association with some historical figure, period, or event. Proof of qualification
of a particular firearm under this category may be established by evidence
of present value and evidence that like firearms are not available except as
collector’s items, or that the value of like firearms available in ordinary
commercial channels is substantially less.
http://www.gpo.gov/fdsys/granule/CFR-2002-title27-vol1/CFR-2002-title27-vol1-sec178-11
Hope this helps.
Jimro