im buyin an 86 chevy full size half ton pickup with dual tanks and a big block 350, planning a 900 mile trip for right after i buy it and need to know the gas mileage, anybody know what one would get?

I wouldn't expect good gas mileage out of it. I really don't have a good estimate to give you but I'd say guess 8-10 gallons in the city and maybe 13-15 on the highway at best. Honestly that is just a guess, but if I were you I'd shoot low and make sure you have enough $$ and fill up locations along to way.

I'll let you do the math.

a 350 is not a big block