Hi,
ACE Gearboxes have been going for well over 20 years, having said that, I have never used them. However a phone call to describe your symptoms to them should get you a rough estimate.
Is the car a keeper, or are you going to move it on in the near future? The cost implications between a rebuild and a used box from a breaker may need to be taken into consideration. Do not forget also to budget for a new Clutch Assembly and Thrust bearing as it would be prudent to replace those items at the same time when the box is out.
I notice also you have a separate thread enquiring about the rebuild of a Back Axle. What other problems does your car have? Would you consider buying a good donor car to strip out the parts? Just a thought if you have the space. My thought processes are that if you have a back axle problem, there may be damage also to the half shafts, and bearing damage. The list goes on and on and the £££££'s mount up.
Steve.