Maybe, but it is a digital tach running off inductive pickup on spark plug wire. I just checked to make sure I had it on 8 cyl. and it was. I will try another tach the next warm day.
Engine ingredients:
XM278H K34-241-5 CompCams Kit http://www.compcams.com/Cam_Specs/CamDetails.aspx?csid=1002&sb=2
1 pair of  C8VEE heads stock valves new guides CNC ported by Red Man
one set of Ferrea 1.76 exh valves
one set of Ferrea 2.19 intake valves
One set of exhaust seats
ARP Guide plates
ARP 7/16 studs
Roller rockers BB FORD PROCOMP T6 ALUMINUM ROLLER ROCKERS 351C 429 460
Smith Bros, Restricted push rods
Ford Racing Tall valve covers
VC cover stud kit ARP 400-7614 stainless 12 point
Arp intake bolts
10/10 crank
All bearings Clevite
Armondo 10 Qt. oil pan
Mellings HV oil pump
ARP HP oil pump shaft
New  dizzy gear and pin
ARP rod studs
Ford truck rods D6VE-AA
TRW 2404 .030 over pistons
Block bored .030 over stock /decked .029"
Oil drain back holes added or enlarged per HyflowDynamics instructions
Dizzy gear oil modification per HyflowDynamics instructions
ARP HP oil pump drive shaft
Performer RPM Intake
1” Carb spacer
Holley marine 850CFM  double pumper  #80443  Jets #88 Front #96 Rear
  2 Power valves 6.5
front squirter .31 rear squirter .50

My 79 CVX 20 is almost the same build and I had similar problems. I have started solving them. My boat GPS today 66 mph at 6100 rpm with an xm278 cam (smaller than yours)  which should be more like 5600 rpm when I change from my A impeller to a AA in the spring. Let's compare notes. I have all my part and machining specs and photos  to send. Kept all my notes and receipts. Have been restoring this for over 7 years. Almost done and almost perfected. Most of your problem is carburetor set up which I had.  Sounds like it is leaning out (running out of fuel) when wot. Wrong jets. power valves and possibly timing. I am in Fort Worth, Texas 8179299829
My email thehondspecialist@gmail.com

Ed k