Will a Bluebird "L" Series 5 Speed Gearbox fit well with an A15 Engine ? in my datsun 1200 ute?

if anyone can quickly give me an answer! cause i wanna buy it now if it does work!

The box will be a tight fit in a manual transmission tunnel, the gearlever will come out just in front of the handbrake, causing problems, & the trans mount is way back, necessitating some structural work to satisfy the registration nazis.

The part that interests me is,... how on earth do you attach it to an A15 engine?

A much better bet is to use one of the gearboxes that were designed to fit to an "A" series engine, or perhaps a Toyotaaargh 5 speed with Dellow adapter.
They are around & should save you a lot of work & grief.