Motor and trans questions Confusion

Hey all. This post is probably going to confuse everyone.... I have some questions.... Hope you guys can help. We'll, we are doing a refit on our 1981 sea ray 310. It currently has beat merc 330s in it with velvet drives. They are coming out. I have two small block crusaders I am replacing them with. I have some concerns and I think the information is correct as far as what I'm posting about the mounts, bolt patterns and stuff. If I'm not someone please tell me what other concerns I have doing this swap..... The replacement engines are crusader 2003 mpi 350s. I am confident they will do the job adequately. Anyway. My mercs are counter rotating, the crusaders are both standard rotation. Which let's me know I cannot use my velvet drives. I believe the bellhousings are the same on a bb and a small block, as are the motor mounts. I have the ecm and harness with these engines. They were originally bolted to hurth transmissions. I also have a Lead on buying a set of hurth 63 transmissions. I'm worried about how they will bolt up, the ratio on the transmissions is 2:1 which is what they were originally used with. I need someone who has some experience with crusader and stuff. I want to use newer gauges... I'm worried about about what issues I'm going to encounter because these are fuel injected...... Someone please tell me if I'm even on the right track..... I can do the swap once I have everything in front of me, I just need to know if I'm going to have to fab anything up. I know I'm going to have to get electric fuel pumps also. Help

454 wot is around 4400 I think 350's wot is around 5500 so you might be looking a different prop combinations

Probably, especially since I'll be going to a hurth trans. Everyone I've talked to has told me what I already suspected, that a velvet j drive just can't be reversed. It's not made for it. Unless it's specifically listed on the tag that it's "fully reversing" don't do it. I'm OK with swapping props.

I also have to make sure the guy I'm getting the engines from has the ecm. I can't find one anywhere. They are discontinued.