First truck need ideas people

[77chevyinline6]

This is my first truck 77 chevy inline 6 2wd 125000 miles the only things i have done to it so far is billet grille glasspack exhaust pyle shaft style radio(i didnt want to ruin my dash) a 400w amp and 2 12 inch subs. im debating whether to lower it or not but the biggest issue is the motor. i havent decided whether to beef up the inline 6 or do a small block v8 swap like a 305 i drive this truck everyday so i cant do a big block due to gas milage. do any of yall have any inline 6s? whats your mpg and how fast is it? because mine drinks gas and it pretty slow