I would have thought the cost difference is neglegable between a L16 & it's bigger brothers.
I mean if I were you, I'd try for a SII Bluey L20B and 5 speed
IF it will fit with only a "massage" (aka knockometer adjustment) of the tunnel.
What is & isn't doable?
L series Article in the Tech section of this site This article doesn't mention gearbox fitting dramas (or lack there of) at all unfortunately.