Amending planting holes in clay has been determined to be a bad idea for quite a while now. You give the plants a bowl to grow in, so the roots had rather stay at home in the nicer soil than venture out into the harder clay. Some will anyway, but you create a bad environment when you do that.

I've planted them in pure clay and they do fine eventually.

I assume cow manure won't hurt anything. You're not adding several inches of it and it's likely aged. And you're likely talking about bagged manure instead of fresh, so it's almost definitely aged somewhat.

Mz,
Buy a big bag of composted cow manure from anywhere.
Dig a very big hole, cleamtis roots are tremedously big.
Add all the composted manure, I would use as much as you can do.
Mix the composted manure well in the clay soil.
When you go to plant, MOUND the clematis up above the soil line
a bit, so that the bottom of the clematis is actually above the soil line.
Then mulch well, that is the key, mulch is your friend.
When you add composted organic material to heavy clay soil you
improve drainage. Then when you mound the plant up a little above
soil level, you improve it again.
You won't get a bowl effect if you add alot of manure, it will be fine.
Dont be stingy with the manure and mixing it well with the clay.
Don't add anything else, NOT topsoil or peat. It is not good to add anything other than composted organic materials, like cow manure, composted leaves, ect.
Good luck, I'm sure the roses and clematis will look gorgeous together.

I have purchased Clematis from Brushwood Nursery in Ga and they have the best directions I have seen regarding planting. I am not affliated with this company.
I top dress my plants with Black Kow Compost in the spring.
Make sure the plant is a little below the ground as it has growing buds at the base. If you have a very cold winter, the bud eyes below the ground will re sprout if all the top growth dies back.

HTH

From Brushwood Nursery:
Planting- Since Clematis live up to 50 years or more, you should take the time to plant each one carefully. Water the pot well before planting. This is an important step for any planting. Once you have selected a location, dig a hole at least twice as big as the pot and twice as deep. In areas with poor drainage, consider a larger hole or mounding up from the ground a bit. Mix some of the soil from the hole with a good topsoil and compost if you wish. The exact proportions will vary by your soil conditions. Backfill some of the hole with this mix. Remove the clematis from its pot by first pushing up on the bottom of the pot and then sliding it out sideways into your other hand. Never pull on the vine or its training stake to remove it. If it doesn't slide out easily, you may roll it gently one half turn on the ground while pushing down (be gentle) to loosen the sides. Clematis roots naturally run deep, so you will notice that most of the roots are in the lower half of the pot. You may gently tease some of the roots away from the sides but be gentle.

Place the clematis on the backfill and look at the soil line of the plant and the ground. The clematis should be about two inches lower than the ground. This keeps the roots cooler and provides buds below ground if the vine should suffer a disease or infestation and you need to cut it back. Every clematis we sell has buds below the soil line, but we'd like you to plant deeply for extra protection. Fill the rest of the hole and over the top of the pot's soil line with the mix you made earlier. Don't cart the extra away until after you water the plant thoroughly. Sometimes the settling soil can leave the vine exposed. After all filling and watering are done, you should mulch your clematis carefully. This can be done with composted pine bark or other mulches, but you can also use stones. This is important because clematis need ample moisture to grow their best and mulch can help the soil retain moisture. Also, be sure to go back and water new plantings regularly during the first growing season. This will help them get off to a great start!

Thank you all for your answers. Much appreciated! Chris I do believe in no amending for trees, but for smaller plants I just make sure I overdig a hole. I've always amended soil for my roses and they are my pride and joy. Butterfly, I am glad to hear you believe in cow manure as much as I do. I only use black Kow so roots are not damaged, and I just think cow manure can do miracles in a garden. Others have told me to stop adding topsoil when I ammend so maybe it's time to stop that practice. Lsst, you made me regret not ordering from that company in Georgia! Great advice. I think I'm now ready to plant when they arrive. Thank you again!