Also can get 5/16" plate 4" x 5" drill both mounting holes and bolt to g/box mt, centralise on X member, mark plate through mount holes, either drill or weld plate to X member.

I have a toploader in my Bird but for the sake of clarity it is a '66 trany and it has a R-11 overdrive grafted to it. I think the advice about making a support plate is Ok but there are a couple problems to face with a Bird that pas cars don't have. I am assuming you do not want to cut the sheetmetal tunnel over the trany.  You should drop the small bolt-in crossmember between the legs of the X by the same amount as the thickness of the plate you are using.  Bolt the plate to trany and use the rubber between the plate and the U shaped bracket of the crossmember, same as the original setup.  This is important because there will be shift linkage issues with the sheetmetal that even a 1/4" will make worse.

I could not use the mounting pad on the trany tailshaft housing (before I had the OD) to mount a Hurst shifter as the shifter will then hit the dash and it didn't come out in the existing shifter hole.  I hand fabbed a mount to hold the 9" Hurst I have (which is vintage 1966) in the original location on the car's sheetmetal tunnel.  This has proved flawless over nearly 12 years so I have no problem recommending it,  it is also how FoMoCo mounted the manual shifters in the original Birds.  I also had to refab. the shift linkage from the Hurst to the Trany using Hurst's trany levers and extending the Hurst rods using 1/4" black iron pipe.  There was a lot of crawling under the car to get it all right

I Also had a new driveshaft fabbed which is 3" dia.  3" does not hit any sheetmetal in my bird and is much stronger than the 1 1/2 or 2" shaft that birds come with.