During their hay day Catalina must have used 20 different extruding machines / vendors to manufacture their mast and booms, it is incredibly lucky to find perfect matching mast for your add-on piece which we installed on top of the our mast.
The added length for a tall rig happens above the spreaders.
First, we found the best looking / fitting matching standard mast and add 2' at the "top"
Second, built a T-bar w/ added shim that can be ground down (see attached pic) for adjusting that fit the slug track tight and perfect assuring a smooth hoist;
We then champ down both parts down using two 4" x 4 x 3/8" thick aluminum angle 3-4ft lg on both sides of the mast using hog jaws or 11R Vise grips assuring a perfect line; " do not use angle "iron", this will transfer / impregnate carbon into your aluminum mast and may cause rust before during and after painting.
Slide the t-bar and shim together back and forward as if it were the slide slug, " perfectly smooth transition, Very important!
Third, we purchased 3/16"x 6' - 12" long type 6061 T6 aluminum 3-4" flat bar as wide as will lay flat "inside" both port and starboard sides of the mast, these are the inside fish plates.
We then drill 2ea 1.25" weld plug holes on each side; use a rose bud to preheat aluminum fish plates, bevel the weld joint 45 degrees out approx 1/4" each side and then made one large pass using 5183 filler rod, we then ground down the welds down just a bit to blend nicely. (see attached pic) A 25 year experienced welder will know what to do here".
Be ready to compromise a little quality on the overall finish product due to the mismatching mast.
Once complete, painting the mast will disguise the splice quite well. Awlgrip all the way, one coat per-primmer, two coats primmer and three coats paint , hard first sand 80, second sand 180 - 220 then lightly sand using red scotch bright brand and tack rag between all coats, we used flatterer in all last three coats; wanted to make the mast, boom and spreaders appear as old as the boat. Good luck its a lot of work, simply handling the mast during all this is pretty tough.