Having done this swap a few times with the 430 and 455's I can say out of all the options, an overdrive trans swap is the best and most cost effective. The gear vendors add on unit will not work within the confines of the Riviera X=frame, and may pose a trouble with eliminating the center bearing. I prefer to use 700r4 cores from 87-93 GM cars, mainly corvettes and camaro due to the better stock calibrations. In the past, my total inventment in the trans/rebuild, new convertor and a trans cooler was under $1000. YOu may or may not have to cut the driveshaft, I dont remeber the length of the 400 or 375 but i think they were close. You'll also need to set up the TV cable and adjust it properly....very important! If the engine is bone stock internally, a 3.08 (which i think was stock in Rivi's) is the lowest you should go with a 700r4. a 2.73 will drop the rpm too low in 4th gear around town. 3.08 and 3.23 are excellent. On the upside, the off the line performance is great because the 700 has a 3.06 first gear AND the overdrive will put your 430 at a happy 1900 rpm at 70-ish mph - it was a 700 rpm decrease on the one car over the 3 spd. It also had gains of 4-7mpg dependant on the terrain and speed. SO! parts list 700r4 from 87-93 GM Convertor Lockup kit TV cable and bracket TCI or Trans Dapt Trans mounting plate (mounts chev trans to BOP) Slip yoke and maybe a conversion U-joint Avoid the Holley EFI - it nice if you have one of their exact engine configurations or you're a whiz at tuning EFI cars / stick with tuning the carb and reading the plugs along ith all the other old hand tricks previously mentioned in this post! Good luck!!