If Chassis, engine is TD5 then likely the rest of the running gear is as well. Where is the reverse gear found, up to the left [ 87model ] or down to right ?
If the chassis, engine and running gear are all td5, it is likely the whole vehicle is and therefore the only thing that is '87 is the reg plate. in which case you have a whole world of issues, cannot register unless on a Q as a "new" vehicle and should follow the advice above of getting your money back.
However there is the possibility that everything is 87 other than the chassis and engine which is why I suggested some pics. To put a td5 engine into an '87 chassis is huge task, to fit '87 running gear and body to a new td5 chassis is easier.
Which brings up a separate question is the chassis a new galvanised replacement or is the chassis an old chassis? can you find a chassis number on it, again some pics of the area would be good.
To answer your question about reverse an '87 will run an LT77 which is up and left. A td5 however will have an r380 which is down and right. If it has a td5 engine I would assume it has a later gearbox as well so reverse will be down and right.
Easiest way to get some answers are to put up as many pictures as you can of the exterior, interior, engine bay, and underside as possible.