Probably a gallon or so. I agree it isn't in the best spot for a Spyder. Since the tank is angled forward fuel is in the front when the pickup is uphill near the back.
I was thinking about moving the bung to the front until I thought about it. Under acceleration fuel goes to the back. I've never had a starvation problem, probably due to the float bowls but I don't want to chance it. And I don't want to run a swirl pot either.
What we really need is a custom tank with a small well and baffles that also looks the part.
Put two 5-hole sender flanges on it. One for the wrong fill location and one for the correct. One tank to fit them all.