Hey Tony,
The rectifiers that start with 1N4000 and go to 1N4007 are all the same 1-Amp rated rectifier, but with progressively higher voltage ratings. Pretty sure the 1N4000 is rated at 50 Volts, the 1N4001 at 100 Volts, on up to the 1N4007 at 1000 Volts.
Since they are all used in a 13.5-Volt circuit, any of those numbers will be interchangeable. What any one model radio has in it will be the one the factory got cheap that month. Since any of those numbers has a high enough voltage rating, the difference between a 1N4001 and a 1N4003 is negligible. They will both work exactly the same. So long as the polarity is correct, they will all stay turned off, no matter what the last digit in the type number.
If the polarity of the radio's power is applied backwards, the diode becomes a short-circuit. The normal 5-Amp (I think) fuse for a 2510 will take a short moment for the element inside to melt. During that short moment, the 1-Amp diode gets several Amps through it, while the fuse element gets around to melting. This overheats the diode inside, and melts the silicon junction that made it a diode in the first place. Now that little black part is a dead short for EITHER polarity, until someone replaces (or clips) it.
Now, when a new fuse is installed, and the polarity turned around right, the new fuse blows, too. That's because the 1-Amp diode committed suicide while doing its job. It prevented reversed-polarity juice from reaching all the expensive stuff in the radio, but died in the moment it took to blow the fuse.
If there's any chance you'll get the polarity backwards again, the larger diode will save you the aggravation of having to disassemble the radio and replace the protection diode again later. Assuming that you DO use the correct fuse, and don't put a 20 or 30 Amp fuse in line.
Sure, the original 1-Amp part WILL protect the radio, but only by getting "blown" itself.
In my situation, I worry that a frustrated slip-seat driver will wrap cig-pack foil around the fuse and explode the little 1-Amp diode, leaving the radio unprotected. The larger 6-Amp part won't do that. If you hook up the radio backwards with a 20-Amp fuse, the 6-Amp diode will overheat and short inside, but it won't explode. Might set his pants leg on fire if the power cord is draped across his lap, though.
73