I hope what I offered helps.
Any protection device such as a fuse, circuit breaker, or fusible link needs to be at or as close as it can be at the source of power. Anything between it and the source of power will be unprotected by this device, while anything after it will be protected BUT we have to keep in mind the layout of things and what is connected to it as well as wire sizes and wire lengths.
Just because you can put a 30A fuse on a wire does not mean things are protected. It depends on what the fuse is needing to do as well as what kind of wire is attached to it.
Let's say we have a short length of wire, let's say 12" long off of a battery going to the input of a fuse and then after the fuse, or the output side, we have a 10' wire attached to it that then runs forward and connects to something like a cooling fan switch and then after the switch another length of wire connects to the cooling fan motor.
On this, if we had a short to ground anywhere PAST the output of the fuse, the fuse should blow.
If there was not a fuse off of the battery and ANY wire, cooling switch for the fan or in this case the cooling fan motor itself were to short to ground then at that point of the short ALL the way back to the battery, this wire and/or parts would become red hot, then start melting the insulation off of the wire, then possible melt into yet another wire it is laying against causing it too to then become shorted to ground and it can go on and on until the battery goes dead or the wiring then ignites something in or around the car and then you have a fire.
Another way I have seen things wired, is there is a feed wire off of the battery to something like a master on/off switch and then a fuse is installed after that and knowing that anything before the fuse is unprotected, if the terminals of the master on/off switch were to short to ground, the fuse would never blow as it is AFTER the switch. Some could say, well the studs on the back of a master on/off switch are covered and a short there cannot occur but then look at the switch itself. While it's a remote chance, what if the internal parts inside the switch were to short to ground. With a fuse AFTER the switch and no protection BEFORE the switch, the connections between the switch and the battery will become very hot, melt insulation off of the wire, and possible do even more damage.
While I would like to say that every master on/off switch, any starter solenoid, or any other switch or device will never fail, it could and if we think might what happen in a serious accident, wires can become pinched and shorted.
When you ask the question of "would the 300A fuse be after the battery and before the F solenoid" I have to answer it that the fuse, and it might not be a 300A size, yes would go between the battery positive post and the F solenoid. Anything between the input side of the fuse and the battery positive post is NOT protected. This get's back to the questions I asked a poster on the Chevelle site. He might be running a high torque starter and not need the power like a standard GM starter would require, nor do I know if he is attempting to turn over a high compression motor with a gear drive starter or a regular starter, and do not know what cable size and type he has from the trunk to the starter. Getting back to how a doctor cannot prescribe things properly without enough information, the same holds true here. A 300A fuse might work, and it may not.
Here is yet another style of fuse I've seen and this has the fuse as close as I've ever seen to the source of power which in this case is the positive battery post:
A thing I want to do is see how these cube fuses compare against an ANL and an ANN style fuse as far as operating characteristics. Below is the specs on the one companies cube fuse while below that is a spec sheet on an ANL/ANN style. I have to think that some react like a fast blow fuse, while another has more of a time delay before they blow. It's just how they are designed.
I recently came across this picture of a battery cable that was not protected on an 18 wheeler and it melted the battery also:
Kinda wonder what the repair bill was on that to where if it had a properly located and sized fuse, they would only be out for a new cable, a new fuse, and the labor to replace those things. It looks like they had a considerably higher bill and luckily just damaged what they did.
Another thing to notice is the ANL style fuses are designed for lifttruck and other battery operated systems
Jim