here is what I want to do. I currently have a 74 highboy with a 390 np435,np205 that seen its better days, I am looking to buy a 77 highboy but it has a c6 and a np203 in it. I want to swap the 205 in the 77 also I have a c6 from a 390 laying around can I use that tail shaft on the c6 from the 460 when I do the swap. what other problems will I have?

 

If you have a highboy, then you have divorced transfer cases in both trucks. Having a divorced transfer case, means that you can litterally run any transmission in front of the case. You may just have to use the driveshaft from the C6 equiped truck in the earlier truck, but these trucks are virtually identical, unless......your 77 is not a 77, and it is a 77.5. If this is the case, you will have a married transfer case, and this will require a tailshaft change, or you can find a divorced transfer case, and just throw that up there.
That would be the easiest thing to do.

Hopefully both of your trucks are actually HB's and yours swap will be simple.

As an option, you could also use the front half of the divorced 203, and adapt it to the (possible married) 205 to make a doubler. This is in the case that you actually have a married 205, and really want to use it.
Upi may have to be a pretty avid off-roader to appreciate or even use a doubler, but you do have all of the parts, and the adapter is only about 700 dollars.
Just a thought.

 

they are both divorced but I read some where that the tail shafts on the c6s are different for the 203 205 for the shift linkage. are the tails shafts interchangable between 2 c6's one from a 460 and one from a 390?

 

Shift linkages all together are different since the 203 requires two linkages, and the 205 only requires one, as ,ong as a twin stock modification has not been performed.
If you are only changing the tailshaft housing, and not just the tailshaft, you should be alright. 'If not, it is terrible easy to make a bracket that attaches to the tailshaft housing bolts to accommodate the shifter. Really it is simple.
This swap is as easy as pie to do. KNow that the crossmembers are only slightly different, and you need the brackets that are on the 205 to bolt up to the existing crossmember. There are multiple holes in the x-member, and you have to use the correct ones, or the t-case will be off to one side and the little driveshaft will work at a compound angle and vibrate, so keep it straight.
I just went through this upgrade a week ago. Those things are heavy, so eat your wheaties!
Also know that the drivehshafts have to be a different length, so if you have the ones from the donor truck, use those if you can.
If you need pics, I can jump under the truck and snap some for you.
Honestly, look at the shifter and you can see that you can get creative and make something yourself.
Pretty easy to do.
Also, the twin stick modification takes only a little time to perform, and will require a different set of shifters anyway, and this is a fun modification.