jrowan could not be more right!!! i wish i had read his post before my mast project.
During their hay day Catalina must have used 20 different extruding machines / vendors to manufacture their mast and booms, you'll be incredibly lucky to find perfect matching mast for your add-on piece which should be installed on top of the mast.
The added length for a tall rig usually happens above the spreaders.
A solid sleeve is really a great product idea for a busted mast at the step or below the boom.
I do like your way of thinking.
We attached our add-on piece at the "top of the mast", very little load.
First, we found the best looking / fitting matching standard mast we could.
Second, built a T-bar w/ added shim that can be ground down (see attached pic) for adjusting that fit my 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 could transfer carbon into your aluminum mast which may cause rust, it will rear its ugly head before, during and after painting.
Then 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' long type 6061 T6 aluminum 3 or 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; 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 a bit to blend nicely. (see attached pic)
Be ready to compromise a little quality on your over finish product due to mismatching mast parts.
Once complete, painting your 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 on whatever you decide, ether way its a ton of work!
