I would suggest that you make a fixture like my channel iron piece and use a mockup block without a crank in it so as you position the engine it keeps the crank c/l at top frame level and level side to side--------its easy to measure and slide sideways for centering the crank or for any amount of offset--
Put the engine pretty much as far reaware as you can depending on distributor clearance and bell housing to firewall----------------that will give you the most room for accessories on front altho there may be some allowances you'll have to make depending on where you mount alternator and a/c.