Just came across this page and it may have what you are looking for at a little over $300. Check it out...
Reamer, king pin bushings, mgb 1962-1980. Bush reamer, kingpin. 1970 Mg MGB. Genuine Mg Part - 18G597. Ships from Portage MI
www.englishparts.com
Both MOSS Motors and The Little British Car Company have that reamer listed under part #386-370 at $399 and $344 respectively.
BTW, English Parts site appears to be a MOSS reseller. I know The Little British Car Company is a MOSS reseller as I've bought and picked up many LBC parts from them in Farmington, MI.
Tim Hollister
Trout Lake, MI