I converted my original Marina gearbox to Triumph 1850 with overdrive. Mod was very straightforward. Unbolted the Marina bell-housing and bolted it straight on to the Triumph box (same first motion shaft). The Triumph gearbox, from what I remember was similar in width to the Marina with only slight modification to the transmission tunnel and shortening of the prop shaft. Not sure if you would be able to find one now but you could try Rimmers.!
Chris