Easier to get a K10 rear end or just axle and drum swap to your current housing, otherwise you have to deal with welding in spring perches, shock mounts, setting pinion angle, u-joint conversion, and probably adapting the e-brake cables.
I think the 5 lug to 5 lug adapters are good but I hear the 5 lug to 6 lug are two piece which make them less safe. I would probably consider a 6 lug rotor and 6 lug axle swap before I used 5 to 6 adapters.
I believe the two different springs offered for a c10 was one for a small block engine and one for the extra weight of a big block engine. But I could be wrong.
I was thinking vacuum leak or belt speaking but if it only does it in 3rd gear might be a trans issue if you can't get it to whistle in lower gears under the same load. Just a guess.
I have been lowering these trucks since the 80's and I agree 100% with this advise. In my early years I have done and seen it done the wrong way as well as the results of doing it the wrong way. Doing it right is not that expensive so dont cut corners on this.
Rear cab band is 73-80. Need to see both fenders to know for sure what year it is from.
73-80 has side marker lights, 81-87 dont.
73-78 have right side or dual fuel fill cap, 79-80 have right side or dual fuel fill doors.
81-87 have left side or dual fuel fill doors.
I would lower it the height you want to start with(I'm assuming 3/5in drop) but if you want the "temporary 2in drop in the rear" your much better off installing the 2in drop shackles and cutting a notch in the bed brace.
Your other options are drop spring hangers in the front or moving the...
Sorry kleedus but you CAN tuck 295/50/15 tires on a 10in rim with a 5in back space. Actually you can tuck a really big tire if you are willing enough to MAKE it fit. Lol