I just purchased a 77 k5 blazer with a sm465 4speed,and a 350 im looking to run 37 or 38" tire what size lift will clear them with no rubbing and no fender modifications?Also any idea on a good gear ratio and are the axles strong enough?  

On gear ratio you would want at least a 4.88 and a 5.13 would be better but axles are not up to that tires size and you will be flirting with failure.  

you'll need a body lift to clear those tires w/o cutting your fenders. i'm running 33's with a 4" lift and they just barely fit in the wheel well when it's flexed out. your front axle won't take those tires unless you put some new axle shafts in there  

6" with fender trimming.  1/2 ton axles will not hold up, well not very long anyways.  At least 4.88, 5.38 would be better though,  14 bolt ff in the rear and a dana 60 or a 3/4 ton dana 44 woud hold up.  The 44 would probably have to be beefed for 38's though.  

Even if you beef up axle shaft you still have a D44 housing knuckles and ball joints which are not best choice for 38's.  

Thanks for the info,im prob gona run a dana 60 up front and a 14 bolt with a detriot locker in the rear,any good ideas on a front locker? it will be driven on the street so im thinkin 4.88 gears  

Replacement "automatic" lockers for front should be avoided like the plague. The only locker I would use there is a manually selectable one as a Detroit style locker can cause steering bind and increase strain on front drive axleshafts because ujoints are not constant velocity in turns. True Trax LSD's play well in front drive axles. Osh Kosh plow trucks have been using this type of LSD in their truck (front and rear) for over 30 years now.  

I looked at my axles they are a dana 44 front and a 12 bolt rear im thinkin of just goin with a 6" lift and runin 36" tires and re gearing my axles cause im on a budget,i think the axles will hold up to 36" tires? i mainly wheel in the mud  

Personally I say to 33's is pretty solid with 35's in gray area and beyond that pushing it. If you want 36's with stock axle, do not install a Detroit type locker. With open diff's or LSD's it should last a while.  

ok from experience i have run 36 inch tires with half ton axles and had good luck with it. but the gearing they are telling you to run is all wrong. if you are going to run on street alot you shoul stay with no more than 4.10 and use granny gear to start off and it will be good on gas.had an 85 blazer with 38's and 3.73 with sm 465 trans and got 15 mpg around town with 383 engine. and had plenty of power in mud. but if you can find them cheap get a set of 3/4 ton axles and you will be alot better off.  

You have very low performance standards of you think a 3.73 is good with 38's with any tranny. Your mistaken view is classic were you depend on granny and low range to get by without given a second though to the torque twist it puts on driveline and chassis. I guess I come from a different era and mold as I have owned 4x4's for over 35 years and remember when they came with 4.56's stock and 4.10 was consider the "standard" minimum with stock tires. My old horse of those day never even need low range unless you just wanted to crawl real slow and with old 465's you NEVER needed granny in 2wd  and did not need in in 4wd unless it was soft ground that you were sinking into. Heck you could even start out in 3rd without any fuss too on pavement pretty easy. Today you guy have much lower expectations of your trucks.