I have a 1976 Vintage Viscount that I have re-done a bit (new paint, hardware, etc) The rack I have doesn't fit the frame well. It seems the horizontal distance from the rear axle to the seat post is shorter than what is expected for most racks. I have the rack mounted but even with the top brackets (those that attached near the seat post) in their shortest position, the front of the rack nearly contacts the seat post. The rack is slightly tipped backward. Not only does this look a bit weird, it also throws the bottoms of my panniers forward reducing the space between the bags and my heals. Not good.

I am hoping someone may have some knowledge around the geometry of older frames and could possibly make a recommendation as to a rack brand that might fit better. The other options is to buy and return many racks until I find one that works or I stop trying. I have considered modifying the rack I have if I can gain access to a welder that can weld aluminum.

Thanks in advance.

It's hard to give you advice without a picture. But I think the problem is that the rear seat stays are very steep, not giving you much room at the seat post. This happens a lot with tall bikes for tall folks, the geometry is just on the extreme for a rack made for the middle, like the common "triple stay" aluminum rear racks.

Look for a rack that is offset to the rear, to move the panniers further back. The Tubus Cargo, Tubus Evo, Nitto 32R, Nitto 33R, and Nitto M20 are good examples. They are all expensive (because they're very high quality), but you may be able to find a cheaper rack with a similar design.