apologizing in advance if this is the wrong area for this but here goes
I have a 89 k5 5.7 stock ten bolt rear my buddy has a 12 bolt 3.40 posi-loc rear for sale is this a decent up grade and is 3.4 low enough i have 33s on it now but was hoping to up them to 35s soon thanks guys

 

I may be wrong but isn't the 12 bolt a 1/2 ton rear axle ? I got lucky and a friend gave me the axles from a K30 he was junking.

 

I have 3.73's with my 33's and I wouldn't go any lower than that with 33's. A 12-bolt is marginally stronger than a 10-bolt, but if you have to change the gears, that can get expensive.

I do believe that you could get the 12-bolt in a 3/4-ton or get the narrow-perch 14-bolt in the heavy 3/4-ton trucks back in the day. I have the rear from our old '75 heavy 3/4-ton that was a 14-bolt. It will bolt into the same location as a 1/2-ton axle.

 

I agree with Swartlkk, 3.42 is not a low enough gear ratio with 33's or even 35's. Especially with your overdrive transmission. 3.73's would be a minimum and 4.10's or even 4.56's would be great with 35's.

 

itll be hard on the trans. first truck i bought had 273s and 38 inch tires. talk about a dog. i drove it like that for about a year too, eventually switching to 373s.

friend of mine gave me a 12bolt/dana44 combo said they were off a 77 3/4 ton pick up truck, no idea if thats the truth or not. i hate the 12bolts just cause they are a little rarer then the 10 bolts. customer brought one into the shop and needed a axle shaft, i thought one of my 10 bolt shaft from home would fit, nope. took a while to actually hunt one down