I have a few lifters making noise until well warmed up

My buddy's 440 had the same symptoms. Turns out several, like 6, of his original pushrods were bend slightly. You could see that they were bent when you spun them in a drill. We replaced them and the noise went away.

You can often isolate the sound using a 3 foot long piece of auto heater hose held to one ear. Be careful of moving engine parts!

Also, a well-known cam manufacturer advised me to add a quart of regular old automatic transmission fluid to the engine oil and run it between changes until it cleans out the lifter plungers. I've had that work, plus it really cleans the inside of the engine. Its just light weight oil with a very high detergent package and some other additives that won't harm your motor.

What motor is this? a slanty or a V8? Check the rocker shaft for wear, but keep the rocker arms and all other parts in order.

If it is an exhaust leak, order up a set of Remflex gaskets, straighten up the exhaust flange mating face with a long body board and some sand paper.

Congratulations on finding a solid convertible. Most are rust buckets. If you are going to replace the top any time soon, they tend to rust right where the rear of the top bolts to the body. Be prepared to treat that area if and when you have it in to the upholsterer to get the top done. It's one of those spots that you can't really see to well until the rear top restraints are removed.