you do NOT have to cut anything to put a new Comet series20 (or clone) in any 14" wheel Speedway. They were already designed for that system. It drops right in.
Now 1970-1972 Speedway 10" wheel bikes, that's a different story. Because they had a 5" driven. And to fit the Comet 6" driven, yes some minor cuts have to be made. But all the mid-1973 Speedway 10" wheel bikes are set up for 6" driven Comet units, they drop right in (they moved the jackshaft a bit towards the front of the bike.) This is when Speedway made the 10" and 14" models more similar, using the same kickstand (just shorter), same footpegs, same handlebars, same torque converter, same seat... they were trying to minimize inventory to save money.
The 14" wheel Speedways (Blue angel, Red baron, Green horn) ALL were designed for a 6" driven. And most of them got the Comet units too (some early 1972 had Fairbank Morse, but still 6" driven.) So the Comet (or clone) 6" driven and drive system drops right in, no modifications needed.
Using a 50 year old Torque converter is frankly, just silly, and probably expensive (if you can even find it.) It's the one thing that gets the crap beat out of it. Just put in a new Comet series20 (or clone.) You'll have better performance and you can get parts for it!