I think Weldor is saying to bend the new aluminum to match the old caps. I agree - don't try bending those cast caps....you'll regret it. Castings don't bend. From the little I can see in your pic, I'd say you might could cut a notch in the new aluminum to help fit the old caps, too. If you're not familiar with dead blow hammers, buy a small one - 1# - and play with it some. They are an "extremely" useful tool. Mine is over 30 years old now and has seen a "lot" of use.
For the rivets - my recent education: I pulled the center seat out of my 16 ft Starcraft project recently to install a folding grab bar and needed 3/16" steel shank rivets to re-install it. The standard kit puller-pliers gave me fits. Both hands straining with every ounce of strength I had and one set bent a handle, the other finally popped a couple but it was a near thing and I was exhausted.
Looked online for heavy duty rivet pullers and looked in shock at $50+ tools - I only needed to pull a dozen or so rivets. Then found a unit in stock at Harbor Freight in Crystal River for $18 that looked good online, so, since it's only 25 miles away, I went for a look-see. Prob'ly made in China but at that point I didn't much care - the thing is beautifully made and very heavy duty. Even has a catch bottle for pulled shanks. Didn't realize how handy that would be until I actually used it - I like it.
Bought it and brought it home and it yanked those stiff 3/16" steel shanks with ease. What a pleasure. Only downside is that it's a big hunk of iron with handles spread and won't fit down in corners. It'll last a lifetime.