Did they give you a reason why you need adjustable rockers?
Here are a few options for adjustable valve train, that don't require machine work.
According to the Crane Cams catalog:
The '68-'71 engines are equipped with bottleneck configuration rocker studs. Our 99768-16 positive locking nuts permit individual valve adjustment (
https://www.summitracing.com/parts/crn-99768-16).
To conveniently convert non-adjustable pedestal mounted rocker arm cylinder heads to fully adjustable configuration, Crane offers two pushrod guideplate and rocker arm stud conversion kits. 52656-16 for 5/16" pushrod and 35655-16 for 3/8" pushrod. Both have 7/16" studs (
https://www.summitracing.com/parts/crn-52655-16 and
https://www.summitracing.com/search?keyword=Crn-35655-16).
I got home late last night and wasn't able to reference my catalogs and downloading/viewing PDFs on my cellphone is a PITA.
I haven't researched the intricacies of Ford camshafts and cylinder heads the way I have with Mopar and Chevy, so I suggest having LakesOnly (
http://highflowdynamics.com) spec you a camshaft and, of course, follow his recommendation for rockers.
The thing I really want to emphasize is: LOSING AN ENGINE FROM BROKEN VALVE TRAIN FRAK'N BLOWS! As bobjthomson emphasized, it's better to spend the money on peace of mind. You don't want to lose an entire engine because a $0.99 rocker stud broke.
With that being said, looking through Summit Racing at CompCams Extreme Marine cams, none seem to be what I would call big. I'm not suggesting any of these cams, just posting for reference only.
https://www.summitracing.com/parts/cca-34-245-5https://www.summitracing.com/parts/cca-34-236-4https://www.summitracing.com/parts/cca-34-232-4https://www.summitracing.com/parts/cca-34-241-5https://www.summitracing.com/parts/cca-34-254-5Since we are talking about a cruiser, I'm going assume the smaller side of the spectrum. Regardless of which cam you choose, your valve train geometry needs to be checked. There are a number of physical limitations as to how much cam with a given head assembly. Let's limit it to just the rocker/push rod for this conversation.
First, the rocker needs to have enough range of motion to accommodate the amount of lobe lift without without binding on the rocker stud. The Chevy guys would call this a long-slot rocker.
If the block and/or heads have been decked, thinner head gaskets, small base circle cam (regrind), ground valve stems, etc. requires shorter push rods. Seating the valves deeper in the head might require longer push rods.
For the most part, I subscribe to Mid-Lift rocker arm geometry (
http://www.mid-lift.com/INTRO-TECH.htm), and this is where I start. However, you can manipulate the effective rocker arm ratio by shortening the push rods, either inadvertently or by design. To see what I mean, watch "More Power Through Improved Rocker Arm Geometry" on YouTube
Adjustable rockers can give you control over lifter preload, but now you can see that if the tolerances are all over the place, adjustable rockers can help. I do believe that with a mild hydraulic cam, you probably won't need adjustable rockers. I can't say for sure because I don't know the details of your build.