I'm half way through exactly that. If your talking celica to A series, you need to have an adaptor plate machined to bolt up to the celica box and the A series auto bell housing. Depending on wich Celica box your using , mods need to be made to the clutch fork , thrust baring carrier . A new cross member needs to be made , the tunnel needs a bit of a massage , the speedo cable matches up to the Celica box with no mods. A clutch needs to be sorted out , a new spiggot baring needs to be made and a new tailshaft is needed.
Easy enough?
J.