Just finished a HOB P51 with an OS25 in it. She flew pretty good once trimmed out, but it took a serious amount of trimming.

Any suggestions where to start as everything looks pretty square and true from the building process.

The model tends to pitch up quite a bit and pull to the left.

CG is exact match to plans and everything else "looked" fine...

Thanks for any input!

once you have your trims set, adjust your control linkages so you can re-zero your trims and still maintain straight and level flight.
left pull might be a thrust line issue.
check this trimming chart for more ideas...
[link]http://www.palosrc.com/instructors/trim-chrt.htm[/link]

Is the up & left movement proportional to throttle? If so, you need to angle the engine down and to the right relative to where it is now. Put some washers behind the engine mount to accomplish this- make small adjustments.