Help, I broke my truck!!!

[73_K5_girlie]

Here's the deal... I have a 73 K5 Blazer with the original but rebuilt 350 in it. It has run great since i got it, it uses a lot of oil, but i don't ever see any under the truck so i guess it's not leaking. I check the oil everytime i put gas in the truck so it stays full. Yesterday on my way home from work from the time I started it to go home it sounded awful funny, just a little louder than normal. As i got up to speed on the main road about 45 mph, the RPM's are way high, around 4000 and it starts to get really loud. When i would let off the gas pedal it would quiet right down and slow significantly, like it was struggling to maintain 45 mph. I pulled over into a parking lot, turned the truck off, and went to check the oil again, bout a quart low, but not too bad. I went to crank it back up and it acted like it wanted to turn over but couldn't, just kinda growled, i pumped the gas a couple times and it fired up, but as soon as i took my foot off the gas it died again... I tried to crank it again and it madew a squeeling noise very loud as it tried to crank so i shut it back down. I walked down to the gas station got a quart of oil and put it in, by that time the man that works in the apartment complex i had pulled over into had gotten the truck to start... but it was knocking... bad. I limped it the mile and a half home really slow and it knocked the whole way. I have a friend coming over tomorrow to look at it, but i was just wondering if from what i've told you if you guys have any idea what could have happened. Thanks guys