I wasn't aware that Ashcrofts did 5-speed rebuilds.
I guess you could have the torque converter replaced & the valve block stripped and cleaned but I'm not sure how much cheaper this would work out than having the 'box replaced with a remanufactured one. For example, K Motors near me do a replacement for £2300 including a new torque converter and oil cooler
Range Rover L322. There will be places nearer to you that do the same, I'm sure.
I can recondition a 5HP24 ('02-'05 petrol L322) transmission for around £500 but I don't tinker with the GM boxes I'm afraid.