basically its a straight swap for the gearbox, they are the same unit although a driver has an E fifth gear. By replacing it with a GTi box it must be an 8V though. basically the differenc is you have a more responsive fiifth gear but lose economy driving. However if the box aint broke dont fix it as its an easy ish job to change but a pain to set it up
Re big bumpers, If yours is a 1990 G on wards it will have the holes ready for the mounting brackets for the bigger bumpers. Dont just use the smaller bumper brackets as the big bumper is not held well at all and either sags or comes adrift. You can get big bumper side brackets off ebay usually for about a tenner a pair. to take the bracket off all you do is push a pen or small screwdriver through the pin and it just comes off.