what transmission to use with new hemi 5.7

You have A-LOT of choices behind the gen III Hemi, the bell-housing is basically the same as an LA motor minus a single bolt at the top.

Automatics:

904 or 727 both can easily built to handle the torque and HP of a Hemi. No modifications to floor pan. Though you do need to shim the trans .060 as the modern Hemi has a different off-set. Bouchillon Performance and many others sell the spacer. Otherwise it is a direct bolt up.

A518 has an overdrive and as others have pointed out requires floor surgery, though not nearly as much as a RFE545. Does NOT require a computer. Can easily be built to handle the torque/HP of a modern Hemi. Like the 727/904 you should shim trans.

RFE 545 Stock Hemi trans. The truck/SUV trans has the speed sensors built into the trans so it is the easiest to retrofit into a muscle car. Hot-Wire can build you a wiring harness to use the truck transmission behind your hemi. Benefits a five speed with two overdrives. Requires SIGNIFICANT tunnel surgery, along with a custom cross member.

Gear Vendors - You can add an over-drive from Gear Vendors to a 727/904 requires virtually no tunnel surgery. (Okay, you might have to dimple the floor in one sport.) Downside? Pricey $2800.00 retail

Manual Transmissions:

A833 is nearly a direct bolt up simply buy an appropriate flywheel and take a die grinder to the input shaft (must shorten it slightly). With all manual transmission switching to a hydraulic throwout bearing is probably recommended as Z-Bar interference with exhaust is likely.

T-5, TKO transmissions, again since it basically has a small block bell housing using one of these transmissions fairly straight forward. The T-5 might be a little on the weak side for a Hemi, but they are cheap and plentiful. Some cutting of the floor is probably in your future.

T-56, 6060 Six speed transmission bell housing are available from Quicktime, be sure to know exactly what your T-56 came out of, as GM t-56 require a different bell-housing than a viper t-56. Either way, major tunnel surgery is in your future, including a custom cross member. Benefit two overdrive gear ratios. Price is dependent upon your ability to score a bitchen' deal on a transmission.

Passon-Performance 5 Speed. If it is ever released this is probably the ideal set up. No cutting of the floor, far stronger than even an 18 spline hemi A833. Only downside is that it is going to sell for $4295.00. That for the transmission and shifter. Bellhousing, flywheel, clutch, etc. Extra. According to what I've read so strong Passon bent the floor of a car in testing, but didn't break the transmission.

Gear vendors, like the autos gear vendors makes an over-drive for the A833, only downside is price, and it does make the trans quite long.

Last but not least you can spend 5K on a trans from Jerico.

Regards,

Joe Dokes