6=8 and bore over?
If you decide to rebuild the slant, it might be a good idea to diagnose it properly.
If it runs good then dies out after 35 minutes, then a rebuild may or may not solve your problem. It sounds like another factor is the problem. If the engine needed a rebuild, it more likely would never run decent at all.
Does it smoke, have you done a leak down test??
Can you set the timing accurately??
A rebuild or engine replacement sounds like it might be a shot gun diagnosis