what did i do wrong? 63 c10 axle swap with 71 not working out

[neverwinter]

i really tried to do a ton of research. from what i read the 71 or 72 rear ends from c10s were the best donors due to using coils springs and trailing arms. i assumed that it also meant the axle perches would be in the same spot and that the panhard bar mount would be close. either i made stupid assumptions or somehow my stuff is way off.

i have a stock 63c10 longbed 4-speed floor shift 283 with the original 12 bolt rear. i wanted to get a better gear ratio and 5 lugs so i looked for a new rear axle.

i had a 79 c10 rear axle but it would need the kit from speedway to get the panhard on there and to weld the axle perches. no problem but then i found a 71 c10 near, me so i grabbed that one. so i got everything from the 63 and the 71 apart.

tried to install the 71 rear axle onto the 63 trailing arms and the axle perch pads arent close to the stock locations - meaning i will need to get the stock perches removed and weld in the new ones from speedway anywa. and the panhard bar is way off.

can someone tell me why people recommend using the 71 or 72 rear axle if it takes the same work as making anything else work? just curious as i was told numerous times to try and find a 71 rear as it drops in - when mine clearly doesnt. just trying to sort where i went wrong.