67-72 vs.73-87 ways to tell chassis apart

[XLR8N]

if you look in the 4 x 4 post under rrrrrrrrrrrrrrrrrrr you will see i had a hard time of installing a new lift on my truck over the weekend. 67-72 front 4 x 4 leaf springs im told are 42'' (unloaded) and 73-87 are 47'' eye to eye. then he tells me that 67-72 rear u-bolts are square and 73-87 are rounded. (mine are round) so now im all messed up on what year truck i have. I have a 67 cab with a 70-72 front clip. is there a way on the frame to be sure of chassis year. i am going to drop some money in this truck and it would make for less headaches on the weekends and having to buy things twice while they hold my money any help in this area would be great!!!!!!!!