Welcome. Being new to the site, you need to be sure to read the "Read Me' posts at the beginning of each forum. The ones here for example recommend that you just post a general introduction here and post tech questions in the appropriate tech forum. This will give you the best chance for a good answer.
Also, those help posts inform you that all caps is not good web etiquette since it is the equivalent of yelling.
If the straight 6 is a 55-up model, by all means keep it. If it is earlier (can tell by the valve cover - early undesirable ones have two nuts holding the valve cover on while the later models use machine screws around the edge of the valve cover). Reason is that the early model engines use splashers to oil the rod bearings whereas the later model have fully pressured oil system to all bearings. The transplant of the later engine was a common occurrence so yours may have it already. Direct bolt in. I am going to run a hopped up late 235 in my '53 Chevy pickup.