I own a 40 Chevy with a GMC 302 Inline 6 running a mild cam.
I am interested in using hi lift rocker arms, or high lift roller rocker arms
Anyone know of a source?
Anyone ever tried to make them from scratch?
Any other engines with roller rockers adaptable to the GMC?
Anyone ever tried to increase the stock rocker ratio by welding on
added length?

Does the GMC head use individual rocker studs or shaft mounted rockers?

Much of the answer depends on which type of rocker mounting system is used and how much work you're willing to do or to pay for.

They are shaft mounted. They are a one of a kind, nothing adapts. RAS used to make roller rockers for unique engines like these. Try them.

Harlan Sharp made me two sets of one-off, custom roller rockers for my crossflow head with shaft mounted rocker arms. I had to supply prints, and I got lucky because they had some extrusions that could be made to work, so I did not have to pay for new extrusion dies.

Pricey, but I had little alternative.

According to Bill Fisher, the GMC intake rocker can be used to replace the 235. It's not an exact match, I assume he means the shaft OD matches, (probably) similar ratio (1.477:1 in the 235), and the offset and lever lengths are very close.
Different: not very strong, difficult to find NOS.
Since the GMC valves are vertical the rocker is a "see-saw" with a line drawn through the pallet radius and the adjuster ball is 90° to the valve. The 235 has a 3°26' (3.433°) angle to match the stem tilt. IMHO this is too small to worry about, and may actually help with certain cams.
The GMC exhaust rocker is very different since the 235 tilt is much larger at 16°.
The intake rocker are far more important, more lift on the exhaust is nice but not as valuable.
The cast (not stamped) GMC rocker can have the ratio increased by brazing up the adjuster thread, and making a new (smaller) hole closer to the shaft. It can't be too close or the pushrod won't clear its relief hole. Any other change requires moving the valve, the shaft, or both.
Just a guess based on the 235: you may be able to get 1.7:1.
A rocker can be made from steel plate, it's not rocket science.
The problem with custom is that it's much larger than any pushrod rocker (except some trucks?) at nearly 3-1/2" from pallet to adjuster (over 1" longer than a BBC rocker) made in the last 50 years TIKO, so a core is not easily adapted.

Arias made them in the past, but my catalog is dated 1999.

Will Willis

From Arias: "we don't make rocker arms".