I made access holes in the boxing plate... enough to get my fingers in and start the bolts and get a wrench in to tighten... Tom
you could use a nut sert maybe... http://compare.ebay.com/like/130657159781?var=lv<yp=AllFixedPriceItemTypes&var=sbar
Use a step drill to chamfer the hole,... use a flange nut and drop it in the hole upside down..... weld it up...... These guys.... Or weld nuts into place before you weld in the boxing plates.....
This thread gives me hope... I thought I might be the only one who excitedly boxed my Model A frame, and then realized I hadn't given much forethought to mounting holes and accessing them!
try threaded bungs. http://www.bungking.com/bungs-5/threaded-bungs.html http://extremecustomparts.com/i-2875560-1-2-13-threaded-weld-in-insert-bung.html
I just made 6 simple brackets on the inner frame rails. I also used swag bar bushings as body bushings. Kind of hard to see in the pic but it is all I have.