One thing I’m confused about is the sending unit. Can you have too much hp for a sending unit or only the fuel pumps? Also I don’t think a single walbro pump will be enough with it? Some people I’ve seen running dual pumps
Please re-read the 1st paragraph of my last post, and I'll explain further. The pump requirement for E85 will be somewhere between 30-50% more volume to make the same HP as with gas. This due to the energy content, or lack of, for E85. Hence, the requirement for much more volume (pump). The biggest Walbro intank pump you can buy will be coming up short of 600 crank HP (500ish wheel HP) with E85. Hence the reason for two pumps or a large pulsewidthed pump.
If you try to run two pumps in the stock tank you're going to find that you'll be hacking the crap out of the tank to get both in and functioning along with the sending unit. Also, the 3/8 line size will get you to your 500whp goal with gas (barely), but will be undersized for E85 at that HP level. At 800 wheel HP on E85 you'll need 5/8" (#10) line. Installing that size line in to the stock sending unit will be almost impossible without cutting a hole in the truck for access.
I seen you mentioned you're looking to use the factory harness and ECM. There isn't a provision to run two pumps individually with that. But you can have one of the pumps run via Hobbs switch or aftermarket switch that will turn it on at 'x' amount of boost.
I do understand the concern for using stock parts, especially for appearance, but you could really use a sump with external pumps. Or a cut, weld, refrabicatation of the stock tank and sending unit. 800wheel is going to be 900+ crank - that's ALOT of go. I'm sure you can get this fuel system done on your budget of $1000-1200, but I'd recommend going at it differently.
Good luck - Jim