I thought G&P gearboxes were crap because of the gears stripping easily or something.
Anyways. OP, keep in mind that if you're shooting 370 on .25's you'll likely be considered a "sniper" and as a result will either have to shoot semi only or pin your gun to semi somehow (either using a MOSFET, or physically blocking automatic fire). Also the MED will be stricter, around 25 or 30 feet.
It's much better to be shooting 395 on .2's and using .25's (you'll be getting ~355-360 on the .25's) that way you won't be considered a sniper and you'll be allowed automatic fire if you need it and your MED will be around 15-20 feet.
I don't know how much mechanical knowledge you have but the other option could be to assemble the gearbox yourself (unlikely since you said you're wary of cracking open the gearbox). Just get a shell and stick with one aftermarket company for parts.
The drop in mechbox you should be getting is probably the SystemA Complete Mechabox (includes motor, $260 USD + free shipping from RedWolf).
Don't bother with the Energy or Revolution gearboxes cause they're way over your budget (with the energy it's $260 USD however doesn't come with a motor).
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE