I'm thinking it can't be done for A/A+ reasons, but a 2nd hand refurbishing one sounds easier than fixing my tired box with buggered synchros.

TIA

Sure, it will work.
You need to run the right primary gear for whatever idler you are using (A or A+), and the box `might' need a little grinding in a couple of spots to clear the rods, but that's it.
No big deal.

However being a 3 synchro (I assume `tis) it's not as strong as any 4 synchro one, and the ratios are crappier too.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.