th400

i have the opportunity to buy a th400 and need to know if it would hold up behind a warmed over 454 and 40in. tires in stock form?  

How well/long it will hold up depends not on the warmed up 454 but rather the gears you have in axles driving those tires. If they are tall stock gears with 40's, the tranny may not live long (and yes even a 4.10 is considered a tall stock gear with 40's)  

as of right now i just have the stock factory 4.56 gears. but i dont want to go too low because it will be my daily driver. and since i live in south georgia the law dont care what size lift or tire you got lol  

What year is the truck and how is it powered now?  

its a 1974 k20 with a 305(not factory of course).  

Okay I will bet performance is not too exciting with 40's and only a 4.56. If MPG is a concern (and it is not going to be good with 40's anyway) I would suggest that you keep 305 and install some 5.38 gears. It will run a lot better and still get better MPG than 454 setup and drive train would be basically bullet proof with that engine and gearing. (it is not as deep as it sound because a 5.38 is like about a 3.73 with stock tires and MPG might actually increase a bit too because you will be on power wasting converter stall less in town) A 305 is not a bad more, just weak on torque but gears can make up for that.  Then when you have some coin and time, look for a bigger SB and a OD tranny to go with the 5.38 gears and you will not be unhappy with the added performance with bigger SB with 5.38 gear or the more civel cruising in OD too. It will also cruise at a much lower RPM with 5.38's in OD than it will with 4.56's with a old non OD tranny.  

it only has 33's right now but im planning on 38's or 40's. but the sm465 helps with the torque issue. i have thought about getting a th700r4 with a monster rebuild kit or installing an OD unit with a th400. but i really would like to have the BBC power and as long as i can keep the mpg in the double digits i wont complain to much because college is only 15 minutes away. as of right now i gota get the carb tuned so im driving my toyota 4x4 untill i can get it sold where i'll have some money for the chevy.  

Forget about double digits with a BB and 40's and a THM400. Do not kid yourself. YOu would have a good chance at it with a SB though. Also, what kills a 700R4 is big tires for gear ratio in use because they have a very tall OD gear in them. No need to do a monster rebuild on one. The trick is to put some real seriously deep axle gears behind the 700R4. That is the key to long life for it.  

as far as mpg my dad had a 1978 k10 with a 350 and th350 with 36's and he still got 16mpg so thats what im going off of and people i've talked to. i wont do anything withgears until i can get me a bigger front axle to handle the tires and motor.  

I lot of people claim to get a lot better MPG than they actually do. When you lift and add big tires you increase rolling resistance and aero dynamic drag. That means more fuel. I have never since a lifted truck get good MPG but you will always see claims of it. WIth 40's and 454 with a THM400, figure on about 7 to 8 in town and maybe 10 on highway with a tail wind if you are lucky. In my 2000 K3500 truck I loose about 2 to 3 MPG off my average  when I go from smooth street tires to aggressive winter studded tires on all four wheels and that are stock size to. No free lunches here.  

dang thats gotta suck with just stock size. well i would rather have a heavy drivetrain and sacrifice some driveability than to risk having something break and not have a way of going.  

i think ima just go with the th400 since i could get it for round $200 or less. and i'll regear when i get a bigger front axle along with the tires.  

I would not. That lack of locking torque converter is going to cost you 7 to 10% in MPG potential. A THM 400 was a good tranny 30 or 40 years ago but there is far better options today. It is just a question of time before gas goes back to 4 bucks a gallon. This drop will not last long.  

well you seem to know your stuff but as far as the th400 its just readily available and cheap. as of right now i have a sm465 which i like because of the granny gear,strength, and to be 30+ years old shifts pretty smooth but the gear ratios are so wide(6.55,3.58,1.57,1.00)i have to wind it out to keep it from lugging down from gear to gear.  

If you want to stay with a manual, you might consider a NV4500. GM models after 94 have a bit closer spread between the gears (5.61, 3.04, 1.67, 1, .75) and much improved shifting quality too.