First the easy one, ALL A500/42RH transmissions have 2.74:1 low gears. Second, I believe the output shaft splines are the same as a 727, you may need a different slip yoke.
Now, as to the converter. I'm considering a similar swap, but have only done a limited amount of research. There don't seem to be too many out there doing this swap, perhaps for a reason. I wish I could be more help, but here's what I know. Pretty sure your transmission has a lockup converter, and that limits selection considerably. Your 91 trans should have the "dog" type pump drive gear as opposed to the 92-up "two flats" or "D" type drive. To use a later converter you would need to use a later pump.
I'm not sure who if anyone makes "off the shelf" performance lockup converter. Here's an idea of what OEM type units are out there. http://www.bulkpart.com/Merchant2/merchant.mvc?Screen=CTGY&Store_Code=2&Category_Code=A500tcLUlate Mopar Performance also list higher stall converters part #s P5007291AB (145K) and P5007292AB (166K). Are these anything other than OEM high-stall units in a MP box? Unsure how durable these units are. Probably fine for street and occasional bracket bashing, but not race car parts.
Other places to consult would be SMR Transmissions http://smrtrans.tripod.com/smrtransmissionsintro/index.html or perhaps PATC http://www.transmissioncenter.net/dodge.htm although both seem more oriented towards the 727 based A518/A618 units
It may also be possible to convert your transmission to non-lockup. Off the top of my head I think you'd need to change the input shaft, reaction support, pump and possibly front drum. Not certain if it's doable, but perhaps worth looking into.