I bought a 1977 blazer. Drives straight down the road. It has a 4 speed with a granny. Is there a direct replacement that is recommended? I found this one, https://www.shiftsst.com/1973-1987-k-series/c347. I can order it, but I'd like to see what some experienced folks have to say?

 

What are your goals here? Anything can be made to fit, but it comes down to your pocket book, skillset, and willingness/availability to do the work.

Pretty much anything, including the tremec found in your link, will require modification to stock items to get them to work; potentially including modifications to clutch actuation, cross members, driveshafts, etc... I have not read all that great of complements on the quality of the kit that is in the link you provided either.

What is the drivetrain in your truck now. Is it 4wd or 2wd? Big difference when it comes to the difficulty/complexity level with 4wd being more complex. If 4wd, does it have the NP205 or NP203 transfer case. Since you mentioned "4 speed with a granny low", I'm assuming the SM465 which would have the NP205 transfer case if you truck is 4wd.

Advance Adapters makes some of the parts that the SilverSport kit includes so you may be able to piece things together more cost effectively sourcing them direct from Tremec & Advance Adapters. Advance Adapters is also more likely to be able to walk you through what is needed to make the conversion if the TR-4050 is your choice for the swap. It does look like a nice upgrade from the SM465 if your intentions are to use the truck on the trails or for towing.