A 305 will take the 906 vortec fine. That is a solid head for a 305. Mill them to around 60-61cc for the compression, I did. In my project journal , you can see 906 vortec heads bolt on a 305 just fine. They work well with the series 2 GM v6 beehive springs. Why would you, the guy asked. Better performance and the same mileage, updated technology that behaves well under more driving conditions, tows like a bastard.
Do a clt see if you are holding compression. It is possible the current head chambers are to big to make the cranking pressure.
Last edited by spinn; 11-18-2012 at 11:10 AM.