That is the key..."Overall diameter of the tire".....
There is nothing wrong with wanting a car to handle like it's on rails...The fact you have some excellent goodies from Heidts will make your car handle much better than most first gen's out there.
In the auto & tire service industry when you go up a size it is called +1 sizing. Example a P205/75/R14 which was the radial equivalent to a bias ply tire has a 26.10 and overall diameter "OD" of 26.10"... A P195/75/R14 has an OD of 25.5 and that size was the stock radial replacement on alot of GM cars back in the day.....I run P235/60/R15's all the way around my car which is +1..... That 15" size has an OD of 26.10" just like the P205/75/R14's..... After the 15" size it's called +2 at 26.17" which was the OD on a P235/55/R16 for 1991 & 1992 Z28 Camaro's with the touring package and 305 engine. RS Camaro's also had that touring size with the touring package in Goodyear RS-A tires.... The IROC-Z or any Z28 with the 5.7L from 1987-1992 had high performance P245/50/R16 which had an OD of 25.64" and that's ok because the 1969 Z28 bias to radial equivalent was more like P225/60/R15 with a 25.62" OD and Goodyear made a VRS tire which was designed for oval track racing for the 5.0L Mustang GT & LX of the 1980's & early 1990's that could be used on a first gen.....P215/65/R15 was a very common size on 1982-1987 Z28's, Buick Grand Nationals, Regal, Cutlass etc and that had an OD of 26.0......Moving forward the next + size was +3 at P275/40ZR17 which could be had on 4th gen's through 2002....Even the 1991 & 1992 Pontiac Firebird SLP Firehawk ran P275/40/17 which comes in at 25.66" and is basically the same overall diameter as the 25.62".
The problem as you are probably aware of even with the +1, +2 or +3 and keeping the overall diameter the same is the car itself. We all know a gens 1-4 of Camaro's have different fenders, suspensions, etc. etc. etc. etc... It's going to be very difficult to stuff a P275/40/17 all the way around on a 1969 but there are guys on this site and others who have done it with the correct modifications.
Here is an excellent tire link for you to compare sizes & Overall Diameter "OD" http://www.1010tires.com/TireSizeCal...?action=submit
P225/45/17 will work and it's 25.0 in OD...You don't want to go too small in OD because I have personally seen too small of a tire in OD put more wear & tear on ball joints/suspension systems... I just checked the Tire Rack website..looks like they say a +4 aka P225/40/18 would fit on there and the OD is 25.4" which would be just right http://www.tirerack.com/tires/tires....o&autoModClar=
But if you look at the price in tires you can get a BF Goodrich for $105 a piece in a 17" and for an 18" it is $251 each, which is what I figured...18's would look great and you should be able to put a P245/40/18 all the way around without mini-tubbing it.....But because it's lowered I would go the 17" route and LS1 powered Camaro's were running 17" wheels, (many of them pro-touring wheels) all day long!
P.S. If you have the stock P/S gear box I would go with the 3rd gen IROC set up. The 3rd gen p/s box improves the feel & response due to the ratio's of the box itself. I got mine from Detroit Speed about three years ago. My car sat for 2+ years... I recently got the car back and you'll hopefully see me more in the tech forums if time allows.
Remember that engineers keep the plus sizing of tires in mind as they make styling changes from year to year....Camaro's have generally been around 3,150 - 3,720 lbs and on average 3,400 lbs between 1967 - 2002 with available V8's and options....Engineers know the weight specs & limits when they design the car and the OD of the tire for V8 equipped cars has basically been about the same for 1967-2002. Of course as years went on emissions, a/c, p/w, pdl, radio, cruise, p/s and other options would vary the weight...And yes I was nearly ready to slap my friends upseide their heads when these guys were taking perfectly good newer pickup trucks, lowering them and sticking P195/50/R15 tires on 'em and wondering why their tie rods, ball joints, shocks, control arm busings etc. were junk by 25k 30k on the odometer back in the day....
go w/ the 17's as LS1 powered cars were running 17's and there should be plenty of options for you in a metric bolt pattern pro-touring style and 17's would likely come in at a significantly reduced cost. P235/45/17 should also work....But you gotta measure the offset, backspacing etc as that varies with custom suspensions & wheels. I hope this bit of tire & wheel info history & facts helped you out....