Noisy 66

[bluechevy66]

I have noise in my 66 I need help with. Here's the situation... I pulled a 12ft trailer yesterday with a medium load at around a 70 mile trip. I noticed it bogging down quite a bit on hills and idling funny at stops. But when I got home to unload the stuff, I heard a sort of clanking, where I think, was coming from the rear end... MAYBE. Also, the motor acted like it got too hot... idling and noisy lifters. Here's what I have in it. I have a gm goodwrench 350 crate motor with about 2000 miles on it, I just converted it to a th350 used auto trans. The drive shaft has all new u-joints. The only thing I'm not sure about is the differential... I don't know if it's original or what the gearing on it is. I do know it has a chrome cover, so someone has at least replaced the cover. Now, the tranny is still shifting smooth and the motor starts right up and idles fine since it's not hot anymore. And after driving it today without a trailer, the clanking has gone away. Plenty of fluid in the rear-end. So, my question is, does the clanking while hauling a load sound like it might be something wrong with the differential? Like I said, it's not clanking now, but was when driven long miles with a load. I'm assuming the tranny is ok since it was shifting smooth even while hot... but then again, I know nothing about transmissions. All the oils are full and look great.
Any help would be great!