i was wondering if there was any way to get better milage out of my 1980 custom deluxe that has a 1976 k5 blazer 350 and a granny 4 speed in it ??  i have a 1977 chevy nova straight 6 cyl engine i am thinking about putting in the 350's place but i have 35" tires on it...is there any way to save fuel and still have enough power to turn the 35" tires ?? ???  

First of all, make no mistake, the 35's and lift are taking a chunk out of your MPG. If you have a smogged 350 it has low compression and will not be fuel friendly. As far as the engine swap, if it was a freshly built 250 of 1970 era or earlier I would say try it but not a smog era 250. I had a 66 3/4 ton 2wd with a 250 and a 4sp and a factory 4.57 rear axle ratio (yes 4.57, not 4.56 and it was std then)over 30 years ago and it did a fine job hauling truck around and I even towed a car on a trailer over 400 miles to Tenn with it once without any power issues. It got about 15 on road by itself and I got about 11 towing the car. IF you are serious about MPG, you need to loose the 35's. If you want to keep 35's, run them close to max air pressure and you want about a 4.10 axle ratio with no OD. If you really want to try 250 you have if you do mostly city and off road driving it might gain you a few MPG but you would need to regear axles to at least a 4.56 or deeper for it to be able to run fairly well. If you are not planning on regearing it, do not waste your time with the swap. If you could find a 1970 or earlier 292 six, that would make a nice swap and it would have as much low end torque (or maybe a bit more) than you have now and should get better MPG too because it was designed to be efficient at lower RPM's unlike most V8's.(it hit its torque peak before 2000 RPM, kinda like a diesel)  A 292 with 4.10 gears would run pretty good too.  

thanks for the info.