I bought a non lockup a518 46rh auto transmission from a 1991 Ram 150 (was behind a 318) that will be mated to a late model 6.1L hemi with about 475hp. From searching, the Sonnax "Sure Cure" a518 rebuild kit is a rebuild kit that also addresses some common design issues of the transmission. Cost of the kit is $112 plus shipping. Ill also be using a forward manual valve body.

Are there any other kits that are better or that you would recommend? Also, what other parts should I consider ordering for the A518 rebuild? Thanks in advance

Should I send the torque converter to be modified or should I just buy another converter altogether?

PATC has some kits...https://www.transmissioncenter.com/shop/master-kit-a516618-hp-w-front-rear-od-brake-powerpacks-90-02/

But if you want top of the line stuff to handle a lot of HP, I would talk to SMR...http://smrtrans.tripod.com/smrtransmissionsintro/index.html

But definitely get rid of the plastic accumulator piston

I also read that I should upgrade the intermediate shaft to a later version that had larger ports/holes. Does anyone have a part number or info on the intermediate shaft?

There is absolutely nothing wrong with the standard 46 RH geartrain including the intermediate shaft and overdrive on units built after 1990. Using the ATF 4 fluids or good synthetics go a long way to add durability to the units. Using billet front and rear servo pistons is a good idea. The biggest culprit to early failures on these early units were the converter lockup discs failing and crappy ATF 3.
Don't waste your time messing with opening up passages and such in the OD unit, just use the later fluids in a properly built unit and you will be in great shape. I would avoid using the 3 disc front clutch if your unit has it by upgrading to a 4 disc with the appropriate reaction plate in the drum.

Changing the spring in the OD unit to the heavier one is a good idea.

PATC and A&A are your best friends for 518 parts and advice.

Been a trans builder for over 30 years. I don't post much, but listen to Transman, he speaks volumes.