Hi Bob,
The TR4A, TR250/5 and TR6 frame itself does allow for more flex than the earlier "ladder framed" cars. Partly, this is just the design. Also, it can be made worse by age and condition. For example, the narrow part that the exhaust pipes pass through can rust or crack around the welds the top side, known as the "t-shirt".
One quick, external check of a TR IRS frame is the gap at the rear edge of each door. That gap tends to open up at the top when a frame weakens. It is usually 1/4" or less. If it's 3/8" or more and significantly wider at the top than the bottom, the frame is likely weakening in the middle and "humping up". Hopefully your's is not a frame issue because that can be harder to repair.
I gotta ask, is this the first you have driven the car? Because it would be great if you could compare to another TR6 yourself and/or have someone experienced with other TR6 drive your car and give their opinion.
You might have a couple separate issues... based upon your description. There are any number of things you might check for problems, or do to tighten up the car's handling:
The first is to do just as you suggest... make sure all the body mounting points are secure. Do you have a service manual showing these? If not, look at a Moss catalog or their website, for illustrations of the frame. That can be a good guide to help you locate each mounting point to check that it is secure. Many mounting points use rubber packing and/or aluminum spacers. How many are used are different on each car, since they are used to correct for some manufacturing variances in both frame and body, and to adjust things like door gap.
Next check the brackets by which the lower front suspension a-arms are bolted to the main frame rail (with adjusting shims). Be sure they have not cracked loose from the frame. Later TR6 had improved reinforcing on these brackets, compared to TR4A, 250 etc., but they are still a possible problem area. Also look all around the front suspension towers for any cracks. And, does the car have the cross-brace installed, that bolts from side-to-side in the engine compartment, from one suspension tower to the other in front of the engine and parallel with the steering rack? If not, that would almost certainly allow a lot of front frame flex.
Next thing I'd look at is the mounting of the steering rack and the steering column. The rubber bushings of the rack can be replaced, possibly even with urethane or solid aluminum, to make steering more precise. If replaced, they need to be pretensioned before the bolts or tightened, or the rack will get loose again in a very short time.
The rubber "donut" in the steering column might need to be replaced, too.
The upper column has two rubber bushings inside it, that the steering shaft runs in and that might be worn and causing slop. Also, the actual mounting bracket of the upper column tube can loosen or crack. Look up under the dash for the mount, right at the firewall, to see if it's solid and secure. Can you move the steering wheel from side-to-side while sitting in the car? Any play indicates either worn bushing (steering shaft loose within the column tube) or broken/loose mounting brackets (the whole assmebly moves in relation to the dash).
The steering rack itself has a bronze bushing in the righthand side and an adjustment at the pinion. If the bushing is worn or the adjusment out, steering will feel sloppy. The inner and outer ends of the tie rods that connect the rack to the front wheels can get play in them too.
The next thing is the suspension. The rear of TR6 are a sometimes a little softly sprung, but you said you feel most of the problem is up front so let's look there first. There might be wear in the bushings of the a-arms and/or ball joints. It's possible to replace rubber bushings with urethane if you'd prefer more precise handling and don't mind a little increased harshness and noise. The original rubber bushings don't last very long and are pretty soft.
Also check the roll bar is nice and securely mounted. if one end is loose that can make for some pretty weird handling characteristics.
The rear trailing arms can also loosen at the frame, or at the hub mounts, or the bushings in them can wear. Sometimes wear and slop at rear feels more like it's at the front. Also see that the rear shocks are secure and working well.
You might be able to narrow down steering and suspension problems, to pinpoint wear or problems, by watching it closely while someone else moves the steering wheel and/or bounces the front suspension. It's a little tricky to do this, since the weight of the car should be on the suspension while you are inspecting it. Jacking up the car leaves the suspension hanging at full extension, which won't show wear and tear in the same way as the more usual ride position. A set of ramps to drive the car up on is probably the best way to check these things out, but be careful and safe.
If you can't see anything, you might be able to feel it by putting your hand across a particular joint while someone moves the steering or suspension a little. Just be careful not to get your hand caught or pinched in the mechanism.
You might also find worn or loose wheel bearings or bad front shocks are contributing to the problem.
An alternative to all of the above, have an alignment shop look the car over and give you their opinion. Perhaps it needs an alignment anyway, and there would be little additional cost to have it inspected by a pro.
P.S. Recommended reading. You might want to get a copy of Roger Williams "Restoring Triumph TR5/250/6" I think he goes into various things to check in a lot more detail than is possible here. He also has a separate book on "Improving..." that includes details frame reinforcement techniqes.
Hope this helps!
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