SS Tim provides great information and comparisons in this thread(post #9)... a must read for you.
http://67-72chevytrucks.com/vboard/s...d.php?t=659340
I would run the frame you have, beat it like a rented mule. If(IF) the frame ever cracks... repair it by bolting on flitch plates or an inserted piece of channel iron. Never weld your frame when you can do otherwise. The K frames were designed to flex, in lieu of cracking under stress.
Another two cents... worth the price paid.
Good luck!
j