where abouts are you? must be North America somewhere to use "roadster"
The diesel 720 has an SD20 ad sd23 bellhousing that can be used with any of the 71 series 5 speeds found in rwd Nissan cars the diesel ratios are abysmal so wouldnt use that box as a whole, The R16 is also an earlier version of the h20 and U20 very beefy bottom ends and the reason the diesels where based on them.
The R16 is a heavy motor if all cast iron there was an r16 sss with twin carbs and semi sport headers and if you can put later pistons using the H20 crank and rods then you have a 2L engine which could also be enlarged to 2.3L if bored out.

I like the R16 looks and with some work using the SSS alloy head along with H20 bottom end, 48 webers, roller rockers, tuned length headers/exhaust, ignition and better cam grind than a U20 roadster could shield 150-180hp ate.