'63 Valiant Newbie

I'd probably call dropping the trans (and leaving the engine installed) the best balance of amount of work, ease of work, and likelihood of the job going well on the first try. Beyond that: just follow good practice—use a good brand of replacement parts, make sure everything is clean-clean-clean, make sure the new seal is installed right way round (backwards = guaranteed leak), put an eye on the crankshaft to make sure it's not grooved/worn at the seal surface, offset the halves of the seal relative to the "halves" of the holder (block and lower cap), put a dab of good quality RTV on the cut ends of the halves, etc. But if you choose a good shop, they'll know to do at least most of this on their own.