And for just a relatively few units? Wouldn't happen for financial reasons. I'm sure it would take more costly machinery to make and form heavier tubing. The Schwinn ef bikes are so heavy and bomb proof to start with. I suppose the best way to check would be to completely strip the bike down and weigh the frame and compare with a known regular frame. i've never seen any info on heavier tubing.

Other variations were tabs for tanks and the King Size frames. Normally used for the 3 speed models, I've seen some single speed bikes with the elevated bridge. Those that were intended to be 3 speeds with hand brakes had flat metal elevated seat stay bridges with a hole for mounting the caliper brake and a fork with an bit of extra crown that had a caliper brake hole. There were variants of the standard middleweight frames. Far more $$$$ than just buying a new 7 speed cruiser many of which used a 7 speed igh with built in brake either foot coaster or cable activated band or drum. I can only imagine what a similar conversion would cost now. I used a pair of drum brake hubs on a Typhoon I converted. Most don't have the holes in the fork or seat stay bridge for caliper brakes. The main issue on adding gears to the middle weights is brakes. Schwinn had a number of heavy duty models like the 'Heavy Duty American' and the 'Heavy Duty Wasp'. thicker spokes, fatter spoke nipples and rims with larger spoke nipple holes.

They used the wheels from the Schwinn Twinn tandem. The 'Heavy Duti(y)' designation usually refers to the wheels.

Feb 1973 schwinn heavy duti