I'd like to point out that technically there is no standard "V7" gearbox. All companies do some things a little bit differently.
If you mean the most "TM" compatible then it'll either be TM or G&P (but a G&P M14 will cost you). G&G and CA do slightly different weird things while still being generally V7 compatible. My vote would probably be to get a TM M14 as a base and pick up every G&P upgrade part you can find.
As for which one, either or will do. The Echo1 M14 SOCOM seems like a generally solid rifle IMO, the G&G same thing but since I don't own an M14 and have only seen/handled/played with other peoples' I can't say these things for certain. I haven't done a side by side comparison to a TM but whatever makes you guys' happy.
Get someone to work on your M14 for you, don't do it yourself it's just a bunch of headache and will end up with you bashing your head against the table. Also if you're going to do one thing, might as well upgrade everything else in there as well since it'll take a lot of time to get back into your gearbox and put it together to change out one part.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE