Triumph Spitfire 1500 gearbox query

Yes. I juggled loads of bits between a Dolomite 1850 single rail 'box and a Vitesse 3-rail 'box a couple of years ago with help and advice from this group. Most parts are interchangeable, with two possible exceptions, IIRC. The mainshaft tip on older 'boxes is thinner with a correspondingly smaller bearing and hole in the input shaft end. Older 'boxes have a narrower synchromesh. These differences may only apply to 3-rail boxes, but you might find that you need to use the input shaft from the o/d 'box along with its longer mainshaft.

Don't scrimp on renewing bearings. I need to take mine apart again to replace one that's become noisy.

And don't forget to renew the spigot bearing in the crankshaft before refitting the gearbox.

Other than the mainshaft, there is a big difference in the gear selector mechanism between overdrive and non-overdrive verisons of this box. The selector rail exits the box higher in the overdrive verisons, the internal linkage which achieves this is quite idiosyncratic. There's no great complexity to this, but it's worth studying the operation of the seletcor before dismantling, Cheers, Bill.

I'd check for worn layshaft bearings - if it goes slightly out of 'line' you'll get jumping out of gear, and third will be effected first.

Synchro cones should be easy and cheap to replace.