What torque convertor do I use?

Depends on a number of things.
Is/was your 340 a late '72 or '73 externally balanced engine? Since it's a 904 (not original to 340s in any form) it could be balanced by convertor weights or by the flexplate; which changes the needs for the new engine...
Is your new Blueprint 408 internally or externally balanced? I thought most of Blueprint's engines came through internally balanced...
If your torque convertor and crate engine are both internally balanced, bolt 'em together and go- if not, then even the two external balances would be different (the externally balanced 340 is unique and unlike the LA360 or 5.9 Magnum balance).
@Johnny Mac may chime in and shed some light on BP's balance requirements.