I'm using a fairly recent Electra-X tank on my '60 Redditch (Constellation) framed project. It does fit but on one small point you may have a little work to do. The Indian Bullet tanks fit an all welded frame. including the headstock. The headstock on a Redditch frame uses a casting which JUST fouls the underside of the pressed steel channel around which the tank is formed, right above the tank's front mounting holes! You might have to elongate those holes vertically by 1/8" in order to raise the front end of the tank slightly, so that the front mounting bolt can pass through. I'll also be using a strip of old inner tube between frame and tank at this point to stop ant rattling between the two.
Have fun,
Adrian