For any route, you'll want to have a LU converter. Running in OD with a higher-stall converter (I assume you have some stall) can cause excessive trans temp if you're running below the converter's 'rated rpm'.
A518 can go 2 ways - BH adapter Cbarge mentioned, or an adapter plate that fits between the SB trans and BB engine bolt patterns. It's 1/2" thick IIRC, so requires a starter adapter also and anything else required due to 1/2" difference in location.
If you choose a 518, the early ones are non-LU as Xenon said, then LU + OD, then they became electronically governed (avoid that style).
If you go with a GearVendors, your transmission gets significantly longer, so driveline angles may get tricky, you might need to drop the tail of the trans to help that. Or, putting a truck short-tailshaft setup in to a rebuilt 727 could help.
Quicktime makes a bunch of transmission adapters, including ones for SB or BB Chrysler, that will fit a GM 4L60 or 4L80. The 4L60 is a smaller trans with a deeper 1st gear (like a smallblock A999) and the 4L80 is much bigger, stouter, and with ratios similar to a 727. Aftermarket support for these transmissions is **HUGE**, and either one can be built to handle significantly more than stock. There are $750 electronic controllers to make them function, or you could probably install a manual VB in either one with some electrical doo-dads to handle the lockup converter and OD (like you'd need to do to the A518). There are non-electronic versions of both transmissions, but I don't know as much about them. Later versions of the 4L60 have a removable bellhousing also, which I admire. I have often thought about putting a Quicktime + 4L60 in one of my lower-power cars to get the OD and deeper gears.
You could convert to a manual, and then use an 80s truck 4-sp OD - but its 3.09 1st gear, and the ratio drop to 2nd, might not be so good for a healthy-cammed engine, though.
If you go manual, Passon's has an A833 OD with better gear ration spacing, or they were supposed to be coming out with an all-new 5-speed that would fit in the same as an original 4-speed. That was a few years ago, and last I read they had delays. Don't know if they finished it, you'd need to research. (it's probably $4000, too!)
If you want to get really weird, read about Ford's Borg-Warner 3pd-OD they used in some 60s cars and til around 73 in trucks. Supposedly pretty stout, it allows some gear-splitting like the Gear Vendors, and also does free-wheeling when you let off the gas in OD (I think), which also saves fuel. I've read about this trans a bit recently and it seems really cool.
Overall:
- For many automatic options you'll need to consider:
- floorpan mods (most will need it, but the 4L65 might not),
- crossmember mods,
- a new-length driveshaft,
- consideration to running your speedometer,
- consideration for connecting shifter linkage,
- and how OD and LU need controlled.