There is a new category of this forum titled "Gas Engines." It is specifically for topics relating to gas engines. You may choose to delete this topic and repost it in the gas section. However, the question you ask really relates to all SuperDuty trucks...both gas and diesel. That being the case, you could simply edit the post and don't mention anything about gas! The fact that you mentioned the word "gas" & "V-10" puts it into another category.
In answer to your question, the topic of aftermarket gas tanks has been brought up numerous times before. Try doing a search on the subject and I think you will get the answer you need. I do not think the 38 gallon tank will fit under the SWB unless you remove the spare tire and modify the mounting locations. Maybe someone else could verify this?
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2000 F-250 SD XLT CC LB AT 3:73LS 6.8L
My 2000 SuperDuty