Hi and welcome,
Hey, there are no "low intelligence value" questions! The dumb thing would be not asking questions first!
I think the book you are looking for is one of Roger Williams' "Restoring Triumph..." series. Check Amazon, new or used, generally the best prices are there. I've not got his book pertaining to the 6-cylinder TRs, but the TR4-4A book I do have gives extensive info on body restoration. He has also written a companion series "Improving Triumph..." that spends more time on performance mods.
I can tell you, Williams does *not* advocate cutting the body in half, at least in the TR4/4A book I've got. He discusses it as one possible option and gives some pointers when it might be done and when it can be avoided.
You've recently done another project and are well-versed in the pain, cost and suffering part of it, so no need to warn you.
The TRs through the 6 are relatively easy to work on, probably easier than the B since that's a uni-body car.
All the exterior TR panels such as fenders, hood and trunk lid are designed to be easily removed (once any rusty fittings are overcome).
And, yes, you can lift the body off the frame relatively quickly and easily. Replacement panels are easily obtained through the major LBC vendors, if needed. However, I feel floor and sills, in particular, are most easily replaced with the body on the frame. In fact, I'd call this even more essential on TR4A IRS and later TRs. The frame acts as a jig such as would have been used when it was originally built, and makes fitting the new welded-in panels easier and more precise.
The only caveat I'd mention is that the frame must first be checked for squareness and solidity. You don't want to fit body panels to a sagged or tweaked frame. That's particularly true of an IRS model such as the TR6, which can get "humped" in the middle and cause a lot of fitting problems.
It might be most expedient to temporarily lift the body off the frame; check, straighten, repair and reinforce the frame as needed; then reinstall the body for its panel work. In fact, on IRS cars it is important that the body be close to normal weight and sitting fully on its suspension for panel fitting. There is enough flex in the body and frame to make a difference. TR4 and earlier non-IRS cars are quite different.
If an initial body lift is done to check and repair the frame, it might be wise to leave the final finishing of the chassis until the second time around. A finished frame and chassis can easily get marred by welding around it, or overspray from primer, etc. Plus, work is slower because extra care must be taken around the finished pieces.
Once the body is back on the frame, the best procedure would be to remove a particular panel, fit the new one, tack weld it in place, then go back and finish welding it carefully. The only exception might be if an adjacent panel will also be replaced. Then I'd suggest just skipping finish-welding that side, but leave it temporarily tack welded. A key reason to leave the body on the frame as long as possible and remove/replace one panel at a time is that heating/cooling from welding can quickly warp thin steel body panels.
I wouldn't cut the car in half unless absolutely necessary. That's most likely just making more work for yourself putting it back together. Yes, it can be done. But, a lot of measuring before disassembly and even more while reassembling will be critical to getting good fit. Seems to me much, much easier to just remove/replace one panel at a time.
I'm not sure what you are refering to as a "bulkhead": firewall? footwells? battery box?
You sound like you are formulatinig a plan of action, a sequence of how parts will be replaced, and getting some idea what all needs to be removed to get the space to work. Williams' book will help you decide.
I suggest you keep every part you remove until the job is complete, so that you can compare new parts, reference the old ones.
TRs are "built around their doors", you'll find. So, that's usually the starting point. After the doors, the sills, rockers, A and B-posts are corrected as needed. Then the scuttle/firewall/footwells and floors. The plan of attack changes depending upon what's needed.
You say your posts are in good shape and I hope that's true. But, if sills and rockers need replacement, it's really likely there may be an unpleasant surprise waiting inside the B-posts, in particular. These usually rust from the inside out. Have you removed the glued-on vinyl from the inside of the B-post, near the floor? That's usually where corrosion problems first show up. It also can't be seen from the outside until the fender is removed. The good news is that a batch repair is usually easy on the base of a B-post, since most of it is well-hidden by vinyl or the fender.
When the body is removed from the frame for finish work, it's important to cross-brace the doors. Personally, I think the best way to do this is to bolt a brace from the hoodstick mountings to the windhshield frame bracket mountings under the dash, and then run at least one solid brace across the car, too. This makes it possible to refit the doors temporarily. Which needs to be done often as sills, rockers and fenders are being worked on, to check the fit. Some folks tack weld the braces in place, which is good, too. But, a couple illustrations in Williams' books show those braces inside the door area, preventing doors from being test fitted. To me, that seems a bad idea.
If you haven't welded or done body work before, the internal tub is a good place to learn, because much of it is hidden under trim and outer panels. However, I'd always recommend considering taking some classes at a local community college, to make the learning curve shorter and to be sure you are fully aware of all safety concerns.
Have fun and keep us posted about your progress!
Alan Myers
San Jose, Calif.
'62 TR4 CT17602L