In an auto parts store, go to the lighting and accessories section. You'll find relays there even if they are labeled for "baja racing lighting"
First off, I highly discourage connecting aftermarket equipment to a 30 year old fuse box. Even if the wiring is new, the ACC (which is the incorrect terminal to use in the first place) can only handle about 10 amps MAX before the fuse blows. The average minimum draw of a fuel pump is about 7.5 amps, damn near 10, and just think, that 7.5 amps has to go thru a looooong set of wires before it reaches the battery.
I used 7.5 amps as an example. What does your fuel pump draw???? You should have this info somewhere, either on the pump itself, or in the instructions.
12 ga. wire can handle about 30 amps absolute maximum. I'd use 12 ga. wire for your application--but that depends on the length of the wire run too. Maybe someone else can chime in as I haven't found a reliable source on wire sizes to use for amprage draw. Your fuse would have to be big enough to handle the load, but not so big that it would allow too much current to go thru the wiring in case of a disaster.
In other words..
Let's say the pump draws 7.5 amps. If you used a 7.5 amp fuse, it would blow right away or not long after. Using a 40 amp fuse means that fuse would allow 40 amps to go thru the wiring and pump before it would blow--a disaster waiting to happen. 10 amps is what I would go with in that situation. Enough to let the nominal current thru, but low enough rating to prevent a disaster.
Hope that helps.