So I've done some research. What I need is ACDelco Part 467513.
I found a post on here from a couple years ago that provided a wonderful link to this part on amazon, but amazon hasn't had that part since about then.
As people have said, this is really the holy grail of car parts for this truck. I've searched pretty thoroughly on the internet and can't find a reputable seller with this part. There are a few places to call (apparently there's a Dan's Auto Parts in Montana that had it at some point and a few others), but it's beginning to look like I'm not getting this part.
The common solution is to use ACDelco U7001.
This process requires all sorts of changes that I refuse to do like changing the switch and at least adding another wire to the selector. I refuse to do this. Also, I'm no engineer, but a motor-driven selector seems much more likely to fail and at upwards of $100 for a non OEM part, I don't think so.
On to plan C, if it'll work.
I've found a three-port, single-wire solenoid valve: Standard Motors FV1.
Since 2*3=6, can I just get two of these and simply daisy-chain the wire to both of them?
The only issues I foresee are either a) I don't understand the electrical side of this and the single wire won't be able to operate both solenoids or b) having independent valves is a safety issue--if one valve failed (especially the return valve), would it be possible for one tank to get pumped into the other tank and overflow? For reference, the fuel pump operates at 30GPH.