M38A1 fuel tank cleaning?

[Casey B]

Hi Fellows..
Well I have to start from the begining again and do it correctly and see if the engine will run better. I cleaned and rebuilt the carb...the kit has two different springs for the accelerator pump and short one and a longer one...NOT the the spring for the metering rod. So I used the longer one like it had.

Put an inline fule filter in between the tank and pump. It caught a lot of large particles from the tank.
I took the sending unit out as well the pick up tube and it was fine except the element was loose so things got past it.
Then I see this stuff on the bottom of the tank. It appears to have gone in as liquid but then dried but can be rubbed off with your finger while still wet with fuel.

Any ideas what it is/ was for?

What do you recomend I clean the tank with when I do the final cleaning out and any tips on how to do it efficiently would be great?

Once cleaned I will rebuild and clean the carb, blow the lines with compressed air and new fuel filter.

I was told I should maybe consider an electric fuel pump as the old one has the rubber diaphram which may not be compatable with the alchol in todays fuels? YES/ Don't Bother?

Thanks to all again...!