big block GMC

[JerryIndy2112]

I have a 71 GMC C3500 longhorn with a 402 in it. The stamp on the block says it is a chevelle engine from 1970. The engine runs fine. But, everything under the truck is leaking and wet. Engine, TH400 transmission and even the rear end.
As of now I am rebuilding the front disk brakes. Had a lot of issues with hoses and the like.
When I got the truck I was able to drive it to get tires on it, the old 16.5. So I have a lot to do to the truck to make it my weekend driver. When I got it up to 50 MPH, it was reving really high in my opinion. I have no tach. So maybe this is normal for the TH400. Maybe the transmission is slipping. I have looked at rebuilding the 402. I am getting prices around $2500 to $3500 to rebuild the 402 to stock. I can buy a 454 rebuilt for around $4000 or less. Most of my searches show the 454 as a long block. Is the 402 I have a long block? To put in a 454 in its place will I need to change motor mounts, radiator and things like that or can I pull out 402 and put the 454 right in? I will also need to rebuild this TH400 or get another in better condition. So I am trying to plan ahead on my restoration. There is body work to do. Cab mounts. New wood bed. Enough to be overwhelming.
The 402 while having good torch is mild on Hp. I looked up the numbers on the block, and it was rated at 330 hp in 1970. I am sure that is good for the truck. It sounds good, but would not cook tires at all. Maybe the old transmission. I would like more from my dream truck.
What does the group have to say? What is on my list of options?

Jerry