Join Date: Jan 2019
Location: Sedgwick, Kansas
Carb spacers are a tuning tool. Sometimes they can improve your top end by a few horses, sometimes not. It's all about harmonics. Pulses running through the manifold, and whether the fuel stays properly in suspension, how well it pulls air and fuel through your carb. Cooler carburetor is nice too, if you have to do a hot start. How tall should it be? Well, start with one size, see if the engine likes it, and then try another size. That seems kind of basic, because it is, and that's really exactly how a tuner finds out what the engine wants.
Open spacer vs 4-hole vs tapered? Well, with your carb, running a dual-plane, you're going to want all the signal you can get, I'm thinking. 4-hole makes more sense to me, because you separate the vacuum signal instead of sharing it from side to side like an open or even tapered spacer would. However, on the top end, the open spacer might (again) give you some extra power - probably at the expense of some low to midrange torque and driveability.
Flat tappet hydraulic lifters might be able to spin to 7k if your valvesprings will take it, and the lifters are not pumping up. Hydraulic roller lifters are heavier, and require more valvespring to control them. They don't do well past about 6500 RPMs, so you need to decide if this quest for power requires more RPMs, or if you can find a way to build more torque and keep your revs down at a more reasonable level. If you really want to sing past 7k, start thinking about solid lifters. As you gain in RPMs, you really start to stress other parts too, like your rods, pistons, and bearings. Having a high quality balance job, damper, and flywheel (along with maybe a safer bellhousing?) become pretty important.
IMHO, knowing that you don't want to redo all the guts of your engine in order to support this quest for 300, making more revs is probably not the direction you want to go with this. Finding more torque instead of just making it farther up in RPMS is probably better for your stated goals.