A switching regulator would be indeed the best solution, followed by a linear regulator.
A linear regulator would be the easiest but you have problem with heat dissipation, as linear regulators dump the difference between input voltage and output voltage as heat.
Skip eBay because it's full of chip clones (or fake) or boards made with subpar chip. Distributors like Digikey have pre-made DC-DC converters that can do what you need, just go to the Power Supplies , dc-dc converters section.
Here's some examples:
4.5v - 14v to 0.6..6v up to 10A :
https://www.digikey.com/product-detail/en/abb-embedded-power/NQR010A0X4Z/555-1161-ND/2270975same up to 6A from other maker :
https://www.digikey.com/product-detail/en/murata-power-solutions-inc/OKR-T-6-W12-C/811-2179-ND/2199629or 8v..16v to 0.6v..8v 4A :
https://www.digikey.com/product-detail/en/abb-embedded-power/APXK004A0X-SRZ/555-1141-1-ND/2170682These are super simple, they just need a resistor or two to set the output voltage and maybe a couple of ceramic or electrolytic capacitors on input and ouput, and they turn on by either connecting a pin to ground or to input voltage, so you can add a switch or set it to always on.
If you want to go with linear regulators, keep in mind that a lot of them can only dissipate up to 15w or so, in the case of common packages like to-220 or dpak
You could configure your regulator to output 7v all the time (as the fuel pump is probably designed to work with 6v lead acid batteries, the max voltage should be more than 6v) and then you'd have (9.2v to 7v ) x 3.5A = 2.2v x 3.5a = ~8A.
You can try a regulator like LM1084 which has a maximum current of 5A :
https://www.digikey.com/product-detail/en/texas-instruments/LM1084IT-ADJ-NOPB/LM1084IT-ADJ-NOPB-ND/363557However, you'd probably have to screw it on a credit card sized heatsink (or a metal case that you would then fix to the metal frame), to keep the chip below 150 degrees Celsius ...
So such solution is cheaper, but it also wastes power and produces a lot of heat...