Yes, '71 was the first year for the disk brakes, and with some parts swapping, spindles from the '73-'87 can be installed to the original control arms. Out back, the stock '68 suspension will outperform the leaf spring rear handling wise, or as Scott pointed out, a 4 bar can be easily installed.
For the time, effort, and cost to swap the frame, you would be ahead to upgrade the stock frame, and get a better handling and braking truck, plus you'll have a truck that the frame and body align to each other correctly.
