Hey matt ,, I was going too convertablize an 1800 at one time and checked out how too do it . befor chopping the top you gut the car and weld in 1/4 inch X 2 inch steel along the door sill , from as far forward as you can go to the rear wheel well . This will stiffen up the chasie / I have herd that running two pieces inside the tunnel is a good idea also.
The frame work of a TR3 rag top is the easyest to modify / not to say its easy.
And the one on Ebay / well I have my douts about it being orig. cause the picture of the windshield , there is too much roof left on. dont know for shure, but looks fishy, later, Les