Hi, my first message and enjoy your room. 305 tbi wont start after running a while.let sit for 15 min and good to go. please advise

First of all, welcome. Lots of good stuff here.
Your problem sounds like the classic GM starter solenoid heat soak issue. The solenoid is very close to the exhaust manifold or headers and after it heats up enough times it won't engage the starter. Let it cool off and presto. If you're using the original exhaust manifolds and the starter is also original get a rebuilt starter and it should be fine for years. If you're running headers get a blanket type heat shield for the starter. It makes the starter last a great deal longer.