I have a 63 super 88 holiday was thinking of installing th350 or 400 in place of Slim Jim. Will I have to cut up floor to install these or will it fit. Thanks for your time.

 

Either transmission would be better than a Slim Jim. I'd probably go with a TH 400 since it's a big car.

 

Need to hear more about your car and your intended use for it. Is the Slim Jim it currently has operable? If it is and the vehicle is just meant to be a cruiser, I'd just leave things be. Any "upgrade" would quickly run into the law of diminishing returns in terms of cost/benefit.

No personal experience, but all the reading I've done suggests that a TH400 swap always seems to involve some degree of floor surgery. While a TH350 would likely hold up under the 394's 440 lb-ft if driven gingerly, I haven't run across any of these swaps to make note of fitment issues.

 

I don't know much about a 1963 88, but I would assume that the frame is very similar to my 1962 Starfire if it has a 394 and slimjim.
I have installed a Ross Racing Engines 700r4 behind my ohauled 394 in my 1962 Starfire.
The 700r4 is similar in diminsions to the trans you are thinking about. To get he trans to fit at the right angle there will need to be tunnel work, a new crossmember will have to be made, unnecessary pieces of the trans will need to be cut off and a new driveshaft made.
It's a job, it's $$$ and time consuming..

 

Another tid-bit here, the transmissions your considering here have torque converters and the slimjim dosen't. The transmissions you're considering will not just bolt up to a 394 without some sort of adapter plate.
It goes on and on, if your slimjim works stay with it

 

OK well I will be dropping off the Slim Jim to see if everything is good and will make a decision then. They car is going to be a driver here to there with an occasional burnout. Thanks for the input

 

You are fortunate to find a shop willing to work on a Slim Jim.
They are a completely different ball game versus a TH350 or TH400.
Most important is the adjustment of the throttle to trans linkage .
The procedure is in the 1963 Oldsmobile Shop Manual.