"You're talking about replacing the main fuel tank,yes? Have you thought about adding a small 1 or 2 gallon tank for the FI pump and the main and underseat keep this FI tank full?"
This is on the 58, so the tank system is different than on the CJ.....it only has a small rear tank (no underseat tank). One of the main goals is to get a bigger tank - v8s are thirsty - even if I keep the carb, it still needs a bigger tank. The current OEM tank has only 13 gal of usable capacity, so I can't even go 150 miles on a tank (which is better than the OEM CJ 10 gal tank!). I am hoping for an improvement in mpg with the TBI, but at best, maybe 15mpg on a gen 1 sbc? So if it had 20 gal of usable capacity, that would be 300 miles, but I'd take a range of 250 miles - that is like 3-4 hours of driving on hwy.
Some have suggested a dual tank set-up (which is what I run in the CJ), but its a bit more complicated as the low pressure fuel system can use the same mechanical pump to pull from either tank (just need a valve to switch between the lines. Also, by 1970, the OEM underseat tank in the CJ has a return line, and the rear tank I installed also has a return line, so a second valve is needed to switch the return lines. On the TBI system, return line is mandatory.
Given that typically, TBI pumps are mounted in the tank, i would need two tanks, each TBI pump compatible (which the current OEM tank is not), both tanks with return line option (which the OEM tank is not), and electrical switch to operate the pumps separately. I'd prefer to use an in tank pump.......but it is possible to use an inline pump mounted in the frame rail, and then I could pull fuel from either tank with single pump and valve (just like the CJ). Still would need return lines to both tanks etc. Last option, is use a second tank with built in TBI pump and set-up, use that tank to feed the TBI system, and then use the original tank as a 'supply' tank to then fill the TBI feed tank. In such a case, I haven't really found a good option to to place a second tank under the frame, and it complicates matters with trying to protect it with skid plates etc.