No, it should be exactly 436,184 Bytes (~426KB). I guess something must have gone wrong with the ZIP compression. I've re-made the file and removed the faulty one from Mega. Try this kit, I've double-checked it: [Only registered and activated users can see links. ]
As you've re-flowed the USB connections etc., it seems very likely you have a failed component on the PCB. Maybe you're into tracing such a fault and micro-soldering, it's certainly not worth paying someone to do any electronic repair on it. There's also the chance a damaged USB port can seem ok visually but that's rare, if you want to be bothered you could try replacing it.

PS: I forgot to mention that the SD flash should make a copy of the device's non-vol region (rgn41) into the card's root as "41.bin". Keep a copy of it. It will also remove a gupdate.gcd fw file from the device's .System folder if it exists there. That way you can do any future fw updates via a GCD in a Garmin folder on card, although it's unlikely Garmin will issue any more for nuvi 40_50 series now.