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I plan to use my stock '55 1st series rear end unless someone tells me they really don't put up with much abuse. I may have up to 300 hp up front with my slightly modified 400 sbc and I plan to get on the gas occasionally. Was a posi unit ever made for this rear end? Will something fit from a later model?

You need a later model rear end. If it isn't noisy now, it will be soon. Back when I was in high school and we ran hopped up sixes there were a lot of broken axles in these trucks (and cars).

I suppose I should have asked this question before I bought my rims. I now have two 6-lug Eagle rims worth about $200 and I would like to stay with the 6-lug pattern for this reason. But I haven't mounted any tires yet so maybe Summit will take them back.

Just swap in a 12 bolt 6 lug from a 73 and up leaf spring Blazer or 1/2 ton 4x4, easy swap,a little welding. It'llstand up to any power a small block can make.Back in business , put on your new wheels

Or if you want a stronger axle than the truck 12 bolts (8.875" ring gear and also have pinion bearing problems) look for the Semi-floating 14 bolt (9.5" ring gear) from light duty 3/4 tons. They came with the 6 lug bolt pattern.

Thanks a bunch for the info guys...you have convinced me that I should replace the stock rear end and I'll start looking. Kenny, what is a "semi-floating" differential?

On a semi-floating rear axle the load is carried on the axle shaft itself. On a full floating rear end the bearings mount on a hollow spindle that the axle shaft goes through. The spindle carries the load and the axle shaft just provides the wheel driving torque. You can tell the full floating axles by the large hub that protrudes past the wheel mounting surface of the brake drum and the ring of bolts that hold the axle flange to the hub body. On the earlier trucks most 1/2 tons were semi-floating and anything bigger was full-floating. At some point in the late seventies or early eighties Chevy started using semi-floating rear axles in some of the lighter duty 3/4 ton trucks.

Thanks Eric for the definitions. I had a 1 ton based motorhome that I scavenged parts for my truck (engine, etc.). I believe that rear axle had the "full floating" design and it had more than six wheel lugs. So the true test of my understanding...my '55 stock rear axle is a semi-floating design, right? And if there is a semi-floating design, and a full floating design, does that imply there is a "no-floating" design?

a full floating is not realy that much stonger inside,
it can just haul a lot bigger loads
your best choice is going to be the 12 bolt out of a post 71 4x4 blazer and a few 1/2 ton pickups had them also
there as tons of hipower parts for them
including posi gear sets, disc brakes, hipower axle shafts,
and much more
chevy used the 12 bolt in 450 hp muscle cars in the late 60s erly 70s so
but keep in mind that the car axles are not all the same as truck axles

SoTxBoTi, Since your '55 has the six lug wheels I assume it is a 1/2 ton which means it is semi-floating. I don't think there is a "no floating" design. I'm no expert on this but I assume that semi-floating refers to the outer end of the axle shaft carries weight(i.e. doesn't float in the bearings) and the inner end of the shaft does not carry weight(does float). The full floating axle doesn't carry weight at either end. I would think that the Semi-floating 14 bolt with it's larger 9.5" ring gear would have to be a tougher rear than the smaller 12 bolt unit. Probably not neccesary for your application though. I'm a firm believer in the old engineering axiom of building it three times as strong as it needs to be though. Saves headaches further down the road.

while we are on the subject, and with rear gear knowledgeables...I have a 59 with 5.26 gears, which vehicle can I organ harvest from to get 3.9 (or 4.10) gears? I have the 8 lug wheels on my one ton.