In my opinion a 1.69 on aftermarket
suspension in a stock 5.0 is not impressive. I've gone 1.69 in a full weight stock motor all stock/original
suspension Fox on street tires. Your accomplishment is impressive, I'm certainly not taking that away, but the proof in the pudding reference to the 60' time does not do it for me.
Out of personal obsession and curiosity, what bolt ons did that stock motor have?
You state you are more confident in the car with poly/solid aftermarket arms and that is great. Confidence will always build speed.
I can tell you that I went from stock rear
suspension to solid relocated uppers, solid axle end and poly chassis end (or is it the other way around?)
lower control arms and it rides more harsh, is less tolerant to movement, and skitters on bumps in corners much worse than when it did before. There is no way it handles better. I guarantee my car is not the only one that has suffered in the handling department as a result of such parts.
As for tire width, it's not a stead fast rule that wider tires will make a car handle worse. But there is certainly a point where a tire becomes wider enough (thats funny) that the contact patch shape and contact pressure are compromised. In 1992-1993 Pontiac made a Cutlass Calais W41. Hear me out. The road car had a sure footed stance with some nice wide rubber. The SCCA Castrol GTX sponsored race car wore much narrower rubber because it offered more performance. I remember one of the rags interviewing the race team engineer. He went on to explain that the road car tire size was chosen by the marketing department for visual effect. He noted also that the Z28 and Corvette road cars were equally handicapped with their wide rubber. That was straight from a GM engineer making statements about the products his employer put into the public hands.
Sometimes it's hard to admit you are wrong.