Decided to replace the belts on my 69 bird with 72 pontiac 400. I was able to get the old belts off, the two new ones are roughly 1/2 to 3/4 inch smaller which I figured was likely due to wear and stretching of the old belts. I got the alternator belt on but it is twisted in one area and the p/s belt I am struggling to get on all three pulleys. Any tips, tricks, suggestions on how to sort these issues out would be greatly appreciated

A v-belt wouldn't stretch that much. After loosening the alternator (and power steering pump) and sliding them in to their inside adjustment and the belts still won't go on, my guess would be that someone either changed your pullies at some point or you have to wrong belts. In either case just take them back to the store and ask for a couple that are maybe an inch longer. Take your old ones with you, they should be able to measure them, or just match them up to a new set.

Look at the number on the belts. It should be a 15xx number. The first 2 number is mm wide so 15mm wide and the second number is length. Are they same length?